Chapter1
AnIntroductionandOverviewofBasis
SetsforMolecularandSolid-State Calculations
JeppeOlsen
1.1Introduction
One-electronwavefunctionsforelectrons,intheformofmolecularorbitalsfor moleculesandbandsforsolids,formthebasisforallthegenerallyapplicablecomputationalmethodsfordescribingtheelectronicstructureofmoleculesandsolids.For densityfunctionalmethods(DFTs),thesingle-electronfunctionsareusedtoobtain thedensity,whichthendeterminestheenergyandallproperties.Fortheothermajor groupofelectronicstructuremethods,wavefunctionmethods,theone-electronfunctionshaveadualpurpose,inthattheyareusedtodescribethedominatingmean-field Hartree–Fock(HF)wavefunctionaswellasthecorrelationbetweentheelectrons. Themolecularorbitalsareingeneralwrittenasalinearexpansionofasetof analyticfunctions,thebasisset.Basissetsareingeneraldevelopedandoptimized forindividualatomsandthebasissetforamoleculecomprisesthenthebasissets oftheatomsconstitutingthemolecule.Thebasisfunctionsarethereforeatomic orbitals,andthestandardseparationofatomicorbitalsintoaradialandaspherical harmonicpartallowsustowriteabasisfunctionas
where n , , and m arethestandardatomicquantumnumbersand A indicatesagiven atom.Thecoordinates (r A ,θ A ,φ A ) arethesphericalcoordinatesinacoordinate systemwithnucleus A attheorigin.
Inspiredbytheformoftheboundstatehydrogenicwavefunctions,theinitially usedradialbasisfunctionswereexponentialfunctionsoftheformofSlater-Type Orbitals[1](STOs), J.Olsen(B) DepartmentofChemistry,AarhusUniversity,Langelandsgade140,DK-8000AarhusC,Denmark e-mail: jeppe@chem.au.dk
©SpringerNatureSwitzerlandAG2021
E.Perlt(ed.), BasisSetsinComputationalChemistry,LectureNotesinChemistry107, https://doi.org/10.1007/978-3-030-67262-1_1
Thevalueofthescreeningconstant ζ differsingeneralforthevariousbasisfunctions, butthisdependencehasfornotationalconveniencebeensuppressedintheabove equation.Althoughsuchbasisfunctionswereusedintheinitialcalculationsofthe electronicstructureofdiatomicmoleculesandstillaremuchusedforatoms,itwas earlyrealizedthatitwasverydifficulttoextendtheuseofthesebasisfunctionsto generalmolecules,inparticular,itisverycumbersometocalculatethethree-and four-centerintegralsneededtodescribetheexpectationvalueofCoulombrepulsion betweenelectrons.
Itwasearlyrealized[2]thatthecalculationsofintegralsofquantummechanical operatorsbetweenbasisfunctionsweremuchsimpler,whenGaussian-TypeOrbitals (GTOs)areusedinsteadofSTOs,
wherethecoefficient α againusuallydiffersforthevariousbasisfunctions.Notice thatinadditiontotheuseofaGaussianform e α r 2 ratherthananexponentialform, e ζ r ,theGTOusesthemonomial r ratherthanthegeneralformoftheSTO, r n 1 . However,singleGaussianfunctionsareverypoorapproximationstotheatomicHF orbitals.Tocompensateforthisdeficiency,itwasimmediatelyrealizedthatthis deficiencyoftheGTOscouldbegreatlyreducedformostpracticalpurposes[3], ifthebasisfunctionwasacontractedGTO(CGTO),wherethebasisfunctionisa linearcombinationofseveralGTOs
TodistinguishbetweenthecontractedGTOandtheindividualGTOsofthebasis setexpansion,thelatterareoftencalledprimitiveGTOs(PGTOs),especiallyasitis standardpracticetocalltheCGTOofEq.(1.4)aGTO.
Thefirstpartofdefiningabasissetforagivenatominvolvesthedefinitionof thelargestvalueof forwhichbasisfunctionsareincludedandthenumberof basisfunctionsforeachvalueof .ForaSTObasisset,anenergyoranothertarget functionisthenminimizedwithrespecttothescreeningcoefficients.ForGTObasis sets,theoptimizationincludesboththenumberofprimitivesincludedineachbasis function,aswellasthecoefficients di ,αi makingupagivenbasisfunction.Often, thescreeningcoefficientsoftheSTOsandGTOsarenotoptimizedindividually,but arechosentoconstituteasimple,oftengeometricseries,andthefewparameters definingthisseriesisthenoptimizedinsteadoftheindividualscreeningconstants.
Sincethemiddleofthesixties,GTObasissetshavebecomethe defacto standard forcalculationsonpolyatomicmolecules,andnumerousprogrampackagesusing GTObasissetshavebeendevelopedandareusedbyquantumchemistsaswellas scientistsnotspecializedincomputationalchemistry.Animportantcontributionto thisdevelopmenthasbeenthedevelopmentanddistributionofGTObasissets.The
distributedcodesincludethusGTObasissetsforallthemajortypesofelectronic structurecodes,targetaccuracy,andpropertiestobecalculated.Themostimportant ofthesedevelopmentsareoutlinedinSect. 1.4.
ThedevelopmentofmethodstocalculateintegralsoverSTOshasnotbeencompletelyabandoned,andagroupofveryskilledandcompetentresearchershavedevelopedimprovedmethodstocalculatethevariousformsofintegrals,andsignificant progressisstillbeingreported.However,duetothelimitednumberofresearchers devotedtothedevelopmentofSTO-basedmethodsandtheinherentcomplexityof thetaskofevaluatingtheintegralsofthesebasisfunctionstohighprecision,thestatusforSTObasissetsismuchlessadvancedthanforGTOs.Thus,althoughrecent researchonthecalculationofmulti-centertwo-electronintegralsholdssignificant promise,theresearchisstillfocusedonthecalculationofindividualintegralsata givengeometry,ratherthanonthedevelopmentofgeneral-purposebasissets.
Forsometypesofcalculations,itispossibletocompletelyabandonthecalculation oftwo-electronintegrals perse.Inparticular,forDFTcalculationswithoutexact exchange,thetwo-electronintegralsoccuronlyintheformoftheCoulombterm, whichmaybeevaluateddirectly.Thiswasfromtheonsetincorporatedinthewidely usedADFprogrampackage[4],sotheuseofSTOsforthecalculationofpolyatomic moleculeshaveinthissensebeenwithusfromtheearlydaysofcomputational chemistry.Wewillreturntothediscussionofthisapproachaswellasthecurrent statusfortheuseofSTObasissetsinSect. 1.5.
Forsolidsandmaterialsthatfromanatom’spointofviewareofinfinitesize,the standardchoiceofbasisfunctionsareplanewaves.Aplanewavefunctionhasthe form
where k isthereciprocallatticevectorandisrequiredtoconformtothetranslational symmetryoftheunitcellofthesolid.Ifallpossiblevectors k areincluded,then thebasisiscomplete,butinpracticethebasissetistruncatedaccordingtoakinetic energycriterion,soall k thattransformaccordingtothesymmetryoftheunitcell andhave |k |≤ E trunc areincluded.Theplanewavebasisfunctionsareorthogonal toeachotherandanumberofintegralsofquantummechanicaloperatorsinvolving thesebasisfunctionsmaybereducedtosimpleanalyticexpressions.Inparticular, thetwo-electronintegralsaretrivialtocalculate.Althoughtheplanewavebasis withouttruncationiscomplete,atruncatedbasissetreproduceshighlylocalized orbitalsonlywithlowaccuracyTheselocalizedorbitals,typicallycoreorinner valenceorbitals,arethereforeinmostsolid-statecalculationsreplacedbyeffective potentialsofvariousforms.Wewillnotdiscusssuchpotentialsorbasisfunctions hereingreaterdetail,butrefertotheChapterbyM.Dollinthisbookforamore detaileddiscussion.
TheGTOs,STOs,andplanewavebasisfunctionsareallglobalbasisfunctions inthesensethattheyextendoverthecompletespace.Analternativeapproachis tousebasisfunctionswithonlylocalsupport,thatisfunctionsthatareonlynonvanishinginalimitedregionofspace.Themostprominentofsuchapproachesare
theFiniteElement(FEM),wavelet,andsplineapproaches,whicharerelatedtothe finitedifference(FD)approachesofstandardnumericalanalysis.Theseapproaches arebrieflydiscussedinSect. 1.6.
Inthefollowingsections,themostimportantconceptsanddevelopmentswill bediscussedfortheGTO,STO,andlocalsupportbasissets.Iwilldivertfromthe historicalapproachanddiscussthedevelopmentsandusesofGTObasissetsbefore theSTObasissets.Themainreasonforthisdepartureisthatitisthecurrentstatus ofGTObasissetsthatthedevelopersoftheSTOapproachmustmatchandperhaps bypassinordertogivetheSTObasissetaprominentrolewithinelectronicstructure modeling.Iwillnaturallybebrief,andrefertothevariouschaptersofthisbookfor amoredetailedandin-depthdiscussions.Inparticular,anumberofimportantforms ofbasissetshavebeenleftout,butthepurposeofthepresentchapteristoprovide ageneraloverview,ratherthananexhaustivereview.Furthermore,thedevelopment ofbasissetsisaveryextensiveeffort,resultinginmultiplepapers.Iwill,forbrevity again,onlyreferencethepaperthatintroducesagivenformofbasisset,ratherthan thecompleteseriesofpapers.
1.2NomenclatureforBasisSets
ThereisacommonnomenclatureforGTOandSTObasissets,whichgivesthenumberofbasisfunctionsofthevariousangularmomenta.Thisnomenclatureisdiscussed indetailinthestandardtextbooksofquantumchemistry,includingRefs.[5–7],but forcompletenessthiswillbebrieflyrecapitulatedhere.Inaminimalorsinglezeta (SZ)basisset,thereisonesetofbasisfunctionsforeachsub-shelloccupiedinthe groundstateoftheatom.Aminimalbasissetforthecarbonatomcontainshencetwo s-functionsandonesetofp-functions,shortenedas2s1p.Atthenextlevel,double zeta(DZ)basissets,therearetwosetsofbasisfunctionsforeachoccupiedsub-shell, sosuchabasiscontains4s2porbitalsforthecarbonatom.Triple,quadruple,quintuple,andhextuplezetabasissetsaredefinedinasimilarway,andareinabbreviated formsdenotedbyTZ,QZ,5Z,and6Z.Althoughtheelectronsinthecore-orbitals givelargecontributionstothetotalenergy,theseorbitalsandenergiesareingeneral ratherconstant,sotheyareoftendescribedusingfewerbasisfunctions,leadingto, forexample,valencedoublezeta(VDZ)basissets,whichforthecarbonatomhas theconstitution3s2p.
Whiles-andp-functionsaresufficientforHartree–Fockcalculationsonthe groundstatesoffirst-rowatoms,molecularcalculationsrequirepolarizationfunctions,i.e.basisfunctionsofhigherangularmomentathanneededfortheatomic Hartree–Fockcalculations,todescribethepolarizationofthedensityfromthefield oftheotheratoms.Asonlytheelectronsinthevalenceorbitalsarepolarizedtoa significantdegree,polarizationfunctionsareincludedonlyfortheseelectrons.At theentrylevelofbasisfunctions,theVDZlevel,asinglesetofd-functionsisadded tofirst-rowatomslikeC,resultinginVDZPbasissets,whichthenhavetheform 3s2p1d.BasissetsofTZorQZqualityareusuallycombinedwithadditionalpolar-
izationfunctions,f-functionsforTZbasissetsandf-andg-setsforQZbasissets. However,thenotationforpolarizationfunctionsisnotingeneralcommonacross differentbasissetssocaremustbeexercisedhere.Forexample,thePusedtodenote thepresenceofpolarizationbasisfunctionsisusedtoindicatethatthebasissetcontainsasinglesetofpolarizationfunctions,independentlyofwhetherthebasissetis adoubleorhigherzetabasisset,aswellastolabelbasissetswherethenumberand typeofthepolarizationfunctionsarealignedwiththenumberofs-andp-functions. AVTZPbasisforCmaythereforehavetheform4s3p2d1faswellas4s3p1d.For basissets,wherethePdenotesasingled-setofpolarizationfunctionsforfirst-row atom,thepresenceof2d-and1f-sub-shellsisdenoted2P.Mostbasissetshave additionalandspecializednotations,butthiswillnotbecoveredhere.
1.3TheGTOVersusSTODiscussion
AlthoughGTObasissetshavebecomethe defacto standard,inparticularforcorrelatedwavefunctioncalculations,researcheffortsforobtainingimprovedmethods forcalculatingintegralsoverSTOfunctionshavecontinued.ThisresearchismotivatedbySTOsallowingsignificantlysmallerexpansionsoftheoccupiedorbitalsand theiradvantagesinregionsaroundthenucleiandfarawayfromthemolecule.When thenucleiareapproximatedaspointcharges,theexactwavefunction fulfillsthe nuclearcusp-condition[8]whenthecoordinates ri ofelectron i equalthecoordinates ofanuclei A withcharge Z A ,
FromEq.(1.3),itisseenthattheonlyGTOfunctionsthatarenon-vanishingatthe originares-functions,andasthesefunctionsherehaveavanishingderivativewith respectto r ,itisobviousthatGTOfunctionsincontrasttotheSTOfunctionscannot fulfillthiscusp-condition.AnyGTObasissetisthereforeincapableoffulfillingthe nuclearcuspconditionatthecenterofthisbasisset.
Fortheregionfarawayfromthenucleiofthemolecule,itcanbeshown[9]that theexactdensity ρex (r ) fallsexponentiallyoff,
where I isthelowestionizationpotentialofthemolecule.ForanyfinitesetofGTOs, thedecayatlongdistanceswillinevitablyhaveaGaussianform,soEq.(1.7)cannot befulfilledinanyfiniteGTObasisset.
ThelateJanAlmlöfmadethecounterargument[10]thatSTOshavetwodeficiencies:theyareincorrectaroundanucleioffinitesizeasthewavefunctioninside suchachargedistributionisknowntobeGaussian,andtheyarenotaccurateatlarge
distancesastheaboveasymptoticcannotbefulfilledwhenevertoodiffuseSTOs arepresentinthebasisset.AlthoughtheargumentsofAlmlöfarecorrect,theyare perhapsnotveryrelevantforpracticaluse.
Fromaformalpointofview,boththeGTOsandSTOsarevalidbasissetexpansions.Ithasthusbeenproventhattheexponents(α intheGTOsand ζ intheSTOs) maybechosensobothtypesofbasissetsarecompletebothinthestandardHilbert spaceofsquareintegrablewavefunctions[11]andinthefirstandsecondSobolev spaces[12].Bysystematicallyextendingeitherofthesebasissets,itisthuspossibletosystematicallyapproachtheexactsolutiontotheSchrödingerequationand theexactenergy.However,itisimportanttobepreciseaboutwhatthiscompletenessimplies.Onthepositiveside,thecompletenessimpliesthatwemayformally deviseasequenceofwavefunctions, i , i = 1, 2,...,expandedinincreasingbasis setsthatconvergetowardtheexactwavefunction ex inthenorm-sensethatis i ex → 0for i →∞ andtheenergy E i of i goestowardtheexactenergy, E ex for i →∞.Ontheotherhand,thecompletenessofabasissetdoesnotensure point-wiseconvergencetowardanyfunctionintheHilbertspace.Thus,althougha completeradialbasissetmaybemadeupofs-typeGTOs, e αi r 2 , i = 1, 2,...,it isnotpossibletoobtainasequenceofwavefunctionsthathaveanon-vanishing derivativethatfulfillsthenuclearcusp-condition,Eq.(1.6),asthisequationisthe propertyofthefunctioninapoint,ratherthaninainterval.
1.4Gaussian-TypeOrbitals
AftertheintroductionofGTOsasbasisfunctionsbyBoys[2]andMcweeny[3]for generaluse,itwasnotuntilthelatesixtiesthatthecomputerhardwareandquantumchemicalsoftwarewerereadytoperform abinitio calculationsonpolyatomic molecules.Thisdevelopmentwasaccompaniedbythedevelopmentofthestandard basissets,inparticularbyPopleandcoworkers.Inthefirstgeneralavailableand usedGTObasissets,theSTO-NGbasissets[13],theGTObasissetsweredesigned asleastsquarefitstoSTOs.However,itwasquicklyrealizedthatitwasmoreuseful todesigntheGTOssothattheyminimizetheHartree–Fockenergyandthislead tothesecondgenerationofbasissets,ofwhichtheVDZbasissets3-21G[14]and 6-31G[15]arestillgenerallyused,especiallywhencombinedwiththepolarization function[16].Anumberofotherbasissetsweredevelopedalongthesamelines, andminoradditionsandmodificationswereintroducedtoallowthecalculationof correlationcontributions.Thesebasissetsprevailedformorethantwodecades,not onlyforHartree–Fockcalculations,butalsoforcorrelationcalculations.
Untilthelateeighties,calculationsincludingcorrelationintheformofconfigurationinteraction(CI),coupledcluster(CC),andMøller-Plessetperturbationtheory (MPPT)calculationswerethusperformedusingbasissetsthatfundamentallywere designedforuncorrelatedcalculations.Thestateofaffairisexemplifiedbycalculationspublishedin1985byAhlrichs,Scharf,andJankowski[17]ontheground statesofseveraldiatomicmolecules.Usingwhatatthattimewasaverylargebasis,
7s5p3d3f1gCGTOs,andtheCPFcorrelationmethod,theyobtainedanelectronic dissociationenergyforN2 of214kcal/molwhichis14kcal/molawayfromthe experimentaldissociationenergyof228kcal/mol.Insomesense,thiscalculation exemplifiedthemidlifecrisisofquantumchemistry:usingthebestavailablebasis setandcorrelationmethod,thecalculationsyieldedresultsthatwerefarawayfrom chemicalaccuracyof4kcal/mol.
However,quantumchemistrydidoutliveitsmidlifecrisisandregainedconfidence andpurpose.Thiswasduetodevelopmentsalongthreelines:improvedcorrelation methods,inparticularcoupledclustermethodswithperturbationestimatesoftriple corrections(CCSD(T)),basissetsdesignedforcorrelationcalculations,andsystematicbasissetextrapolationtowardthecompletebasissetlimit(CBS).Ofthesethree developments,thelattertwoarerelevantinthepresentcontext.Thedevelopment ofbasissetsbettersuitedtodescribecorrelationstartedwiththeANObasissetsof AlmlöfandTaylor[18].Thesebasissetsprovidedseveralnewinsightsandstrategiesthatradicallychangedtheaccuracythatmaybeachievedbyquantumchemical calculations.Firstly,thesebasissetsreflecttheunderstandingthathigherangular momentathanpreviouslyemployedwerenecessarytoobtainaccuratecorrelation energies.Secondly,thebasisfunctionswerethelinearcombinations(contractions) ofprimitiveGaussianswiththecontractioncoefficientsbeingchosenasthoseofthe variousnaturalorbitalsofatomiccorrelatedcalculations.Thirdly,basisfunctions withsimilarcontributionstocorrelationenergywereaddedtogether.Finally,byprovidingahierarchyofbasissets,itwasnowpossibletosystematicallyimprovethe accuracyofthecalculation.
TheANObasissetsprovidedasignificantadvance,buttheywereingeneral consideredastooexpensiveastheyemployalargenumberofprimitiveaswellas contractedbasisfunctions.Moreeconomicalbasissetsforcorrelatedcalculation wereintroducedintheformofthecorrelationconsistentbasissetsofDunningand coworkers[19],andtheANObasissetfromtheLundgroup[20].Correlationorbitals haveingeneralthesamespatialextentasthecorrespondingHartree–Fockorbital, buthaveadditionalnodes,sothecorrelationorbitalsforcoreandvalenceelectrons differ.Basissetsforthecorrelationofthevalenceelectronsandbasissetsforthe correlationofbothcoreandvalencehavebeendevised.
Asgroupsofbasisfunctionswithsimilarcontributionstothecorrelationenergy areaddedtogetherinthecorrelationconsistentbasissets,thevariousbasissetshave averysimplestructureintermsofthenumberofcontractedbasisfunctionsper angularmomentum.Considerasanexamplebasissetsforvalencecorrelationofa first-rowatom.AttheHartree–Focklevel,thebasissetincludes2sand1pcontracted sub-shells.Thefirstsetofcorrelationorbitalsforthevalenceelectronsincludes1s, 1p,and1dsub-shells,sothetotalbasissetincludes3s,2p,and1dsetsoforbitals.The secondcorrelationshelladds1s,1p,1d,and1fsetsoffunctions,sothesecondsetof correlationconsistentbasisfunctionsincludes4s,3p,2d,and1fsetsoforbitals.The basissetmaythereforebedefinedintermsofacardinalnumber X ,whichdescribes thehighest valueincludedinthebasisset,sothebasissetsisdenotedascc-pVXZ andincludesX+1s-functions,Xsetofp-functions,...,and1setoffunctionswith
= X .Notethatthenumberanddistributionoverangularmomentaoftheadded functionsforcorrelationareequaltothatintroducedaboveforpolarizationfunctions.
Withthenewbasissets,systematiccalculationsofgeometriesofotherproperties weremade,seeforexampleRef.[21],anditwasamplydemonstratedthatthecombinationoftheCCSD(T)methodandlargebasissetsprovidesgeometrieswithan accuracythatoftenmatchesexperimentallydeterminedgeometries.Energeticsare alsosignificantlyimprovedsothegoalofchemicalaccuracyofabout4kcal/molis achievableformoleculesthataresmallenoughtoallowtheuseoftheCCSD(T)or multireferenceCImethodswithlargebasissets.Theelectronicdissociationenergy ofthenitrogenmoleculecouldthusbecalculatedtochemicalaccuracy[22].The combineduseoftheCCSD(T)correlationmethodwithlargecorrelationconsistent basisfunctionsprovidedthefirstgoldstandardofcorrelationmethods,andthefield ofquantumchemistryemergedasamaturefieldwithgravitasandconfidence—some wouldclaimover-confidence.
Propertiesdependingonenergydifferences,suchasreactionandatomization energies,exhibitaveryslowconvergencetowardthebasissetlimit,andthisreduces theaccuracythatmaybeachievedevenusinglargebasissets.Thus,althoughthe accuracyof4kcal/molwasachieved,itseemedimpossibleoratleastverydifficult toreducethiserrorbyafactorof10ormore,whichwouldallowtheoreticaldatato beusedforaccuratemodeling.
Theslowconvergenceoftheenergieswithrespecttothesizeofthebasisset isconnectedwiththenon-differentiablebehaviorofthewavefunctionwhentwo electronshavethesamespatialcoordinates.Whentwosinglet-coupledelectrons i and j havethesamecoordinates,theexactwavefunctionexhibitsthecusp
wheretheaverageisoverasmallspheresurroundingthepointwithcoalescing electrons.
Thesenon-differentiablepointsinwavefunctionspaceareobviouslyonlyvery slowconvergentwhenthewavefunctionashereisexpandedasanti-symmetrized productsofdifferentiablewavefunction.However,itmaybeshownthatthisbasisset errorconvergestowardthebasissetlimitasasimplefunctionofthecardinalnumber, X + 1 2 3 ,sotheenergy E ( X ) obtainedinthebasissetwithcardinalnumber X is relatedtothebasissetlimit E ∞ as
where A isaconstanttobedetermined.Thisequationmaybeusedtoextrapolate towardthebasissetlimitbycombiningresultsfromtwoormorecalculations.The simplestandmostsuccesfuloftheseextrapolationschemes[23]uselinearinterpolationoftheenergiesobtainedwithcardinalnumbers X and X + 1toobtainthebasis setlimitas
Ingeneral,thissimpleextrapolationschemereducestheerrorof E ( X + 1) byan orderofmagnitude.
Theconvergencefactor X + 1 2 3 isforsinglet-coupledelectronpairs,whereas theerrorconnectedwithtriplet-coupledelectronpairsissignificantlysmallerand convergesfaster,withafactorproportionalto X + 1 2 5 .Amoreelaborateextrapolationschemeisthusbasedontheform
withtheunknownfactors A , B .However,formostbasissets,extrapolationschemes basedonEq.(1.11)havenotproventobemoreaccuratethanEq.(1.10).Theuse ofthesimplelinearextrapolationschemeofEq.(1.10)andthecoupledcluster wavefunctionswithhigherthantripleexcitationshasledtocalculations[24]and protocols[25]forquantumchemistrywhichgiveatomizationandreactionenergies witherrorslessthan1kJ/mol.
ItisimportanttonotethatthesuccessofthevariousGTObasissetsandextrapolationschemesisanindicationthatthesebasisfunctionsgiveenergycontributions thatareclosetotheexpansionoftheatomicnaturalorbitalsinacompletebasis. Theconvergenceofthecorrelationenergytowardthecompletebasissetlimitwill thereforenotbefundamentallychangedbyanotherchoiceofbasisfunctions,suchas STOs.However,thisdoesnotruleoutthatevenmoreaccurateextrapolationresults maybeobtainedusingGTObasissetswithalargernumberofprimitivesorbasis setsbasedonSTOs.
Thesuccessofthecorrelationconsistentbasissetsandthepossibilitiesforbasis setextrapolationhaveledtorenewedinterestintheconstructionofbasissetsthat systematicallyapproachthebasissetlimitfortheHartree–FockandDensityFunctionalTheorycalculations.Jensenhasdevelopedthepolarizationconsistentbasis sets[26],whichinagreementwiththecorrelationconsistentbasissetsaredefined intermsofthebasisfunctionwiththehighestangularmomentum,butwherethe numberbasisfunctionswithlow ,inparticulars-andp-basisfunctions,arehigher thanforthecomparablecorrelationconsistentbasisset.BasissetsforHartree–Fock aswellasDFTcalculationshavebeendevised.Inagreementwithanolderanalysis ofKlopperandKutzelnigg[27],itwasobserved[28]thattheerrorcomparedtothe completebasissetlimitexhibitedexponentialconvergence.Inparticular,theforms
werefoundoptimalforextrapolationoftheHartree–Fockenergyforcorrelationconsistentandpolarizationconsistentbasissets,respectively.InEqs.(1.12)and(1.13), A , B areextrapolationparametersand N s isthenumberofprimitive s -functionsin thebasisset.Jensenargued[28]thatthepolarizationconsistentbasissetsprovided amoreaccurateandreliableextrapolationthanthecorrelationconsistentbasissets, butKartonandMartin[29]showedthatthesingleparameterform
providedaccurateextrapolationalsoforthecorrelationconsistentbasissets.Note thattheextrapolationformulasinEqs.(1.12)and(1.13)involvetwoparameters andrequirethereforeenergiesfromthreebasissets,whereastheformsuggested byKartonandMartinonlyrequirestheenergyintwobasissets.TheHartree–Fock energyisconvergingsignificantlyfasterthanthecorrelationenergy,sotheHartree–Fockextrapolationschemesreducetypicallytheerroroftheenergyofthelargest basissetbyafactorof2,whereastheextrapolationofthecorrelationenergytypically reducedtheerrorbyafactorof10.
ThecurrentstatusoftheGTObasissetsandtheuseofGTOsformolecular calculationsmaybesummarizedasfollows:
1.Veryefficientalgorithmshavebeendevelopedtodeterminewavefunctionsof smallandlargemolecules.Oftenthecalculationofindividualintegralsarecompletelybypassedorarere-expressedusingCholeskyfactorizationorresolutionof theidentity.Derivativemethodsallowingtheoptimizationofthegeometryand, forexample,vibrationalfrequenciesandintensitieshavebeendeveloped.
2.Basissetsforsystematicallyapproachingthecompletebasissetlimithavebeen developedforDFT,theHFandcorrelatedwavefunctionsforgroundaswellas excitedstates.Basissetstargetedforspecificproperties,forexample,magnetic propertieshavealsobeendeveloped.
3.Thebasissetsaresoaccurateandsystematicthatextrapolationproceduresmay beappliedtoestimatethefullbasissetvaluesforthecorrelationenergy,aswell asforHFandDFTenergies.
4.RelativisticbasisfunctionsaimedatdescribingtheDiracequationeitheratthe two-orthefour-componentlevelhavebeendeveloped,allowingsystematicproceduresalsofortherelativisticwavefunctions.
Thedevelopedextrapolationschemesdonotonlygiverisetoimprovedenergies andotherproperties,butdoalsoprovideestimatesoftheerrorsofagivenbasis setcomparedtothebasissetlimit.AlthoughimprovementsofGTObasissets,in particularforcalculationswithveryhighaccuracy,stillareneeded,thebasissets developedinthelastthreedecadeshavebeenoneoftheimportantingredientsfor developingquantummechanicalcalculationsintoamaturescientificfield,whereit ispossibletoprovideestimatesoftheerrorsassociatedwithagivencalculation.
1.5Slater-TypeOrbitals
ThedevelopmentofbasissetsoverSTOsandotherexponential-typeorbitals(ETOs) hasbeenlessextensivethanthedevelopmentofGaussianbasissets.Thisismainly duetothelackofgeneralandaccurateproceduresforcalculatingthree-andfourcentertwo-electronintegralsoverthesebasisfunctions.Althoughthischapterand bookisdevotedtothediscussionofbasissets,itispertinenttofirstreviewthecurrent statusandprogressinthecalculationofsuchintegralsaswellasotherintegralsover STOs,therebysummarizingthesignificanttechnicalandalgorithmicchallengesthat havebeenovercomeinthelastfewyears.
Forcalculationsonatoms,theproblemwithmulti-centerintegralsisobviously avoidedandtheSTOshasbeenextensivelyusedforhigh-accuracycalculations formanydecades.Arecentadditionisthedevelopmentoftheintegralsrelevantfor generalcalculationsinvolvingcorrelationfactorsintheformofHylleraascorrelation functions[30].
Fordiatomicmolecules,therecentdevelopmentsofLusiakandMoszinski[31–34]haveledtoalgorithmsandimplementationsthatallowtheevaluationofintegrals forSTOswitharbitraryangularmomentum.Inadditiontotheintegralsrequiredfor evaluationmatrixelementsofthenon-relativisticHamiltonian,theyhavedeveloped andprogrammedalgorithmsallowingthecalculationofintegralsforone-bodyrelativisticoperatorsaswellasforeffectivecorepotentialoperators.Theintegralsare obtainedwithveryhighaccuracy,12digitsormore,therebyallowingcalculations usingbasissetshavinglargeoverlaps,astypicallyisthecaseforbasissetsforthe accuratecalculationofcorrelationcontributions.
Forthemuch-soughtgeneralizationtothecalculationofthree-andfour-center integrals,theexpansionintroducedbyGill[35]oftheCoulomboperatorintermsof productsoffunctionsofone-electronfunctionshasledtosignificantprogress.Inparticular,Hogganandcoworkers[36, 37]haveusedtheexpansionofGilltodeviseand programalgorithmsforalltypesofintegralsneededforgeneralpolyatomicmolecules andhavedemonstratedtheusefulnessoftheapproachbyperformingcalculationson thedimerofH2 .Themethodhasalsobeencombinedwithsemi-empiricalmethods andappliedtostudychemicalshiftsofbenzothiazoles[37].Unfortunately,tothe bestofthepresentauthor’sknowledge,thisdevelopmenthasnotbeenextendedto correlatedcalculationsofgeneralpolyatomicmolecules.
Anobviouswayofbypassingthecomplicationswiththedevelopmentofanalytic evaluationofmulti-centerintegralsoverSTOsistocalculatetheintegrals,orthecontributionsfromtheintegrals,usingnumericalintegration.Thisprocedureistypically implementedbyfirstexpandingtheproductoftwoSTOsasasumoverSTOsinan auxiliarybasis[4],whichleadstothetwo-electronintegralsbeingreducedtointegralsbetweenthreeSTOsandthesearethenevaluatedusingnumericaltechniques. ThisapproachhasbeenusedformorethanfourdecadesintheextensivelyusedADF programdevelopedofBaerendsandcoworkersforDFTcalculationswithoutexact exchange[4]andlaterusedbyWatson,Handy,andCohen[38]tocalculationsusing Hartree–FockandDFTwithexchange.
Inbroadterms,therearethereforecurrentlytwotypesofmolecularcalculations, whereSTOsareingeneraluse:high-accuracycalculationsondiatomicmolecules andHartree–FockandDFTcalculationsongeneralmolecules.Startingwiththelatter approach,newbasissetsthatsystematicallyapproachthefullbasissetlimithavebeen developedbyVanLentheandBaerends[39].Thedevelopedbasissetsrangefrom doublezetauptoquadruplezetabasissetsincludingpolarizationfunctions.Forthe heavierelements,thebasissetsareoptimizedforarelativisticHamiltonian.Thebasis setshavebeenextensivelytested[39]andtheresultsofthesmallerbasissetsconverge towardthoseofthelargestbasissets.Theconvergencehasnotbeencomparedtothe exponentialconvergenceofHartree–FockandDFTenergiesthatwerediscussedin theprevioussection.Thetriplezetabasishavesubsequentlybeenslightlyextended byWatson,Handy,andCohen[38]andwereshowntobecomparableinaccuracyto standardtriplezetaGTObasissets.ForHartree–FockandDFTcalculations,there arethusgoodandwell-testedSTObasissetsforallatoms,butthesehavenotbeen demonstratedtoprovidebetterenergiesthanGTObasissetswithsimilarnumbersof contractedGTOs.WhetherthislackofadvantageintheuseofSTOsisfundamental orisduetootherapproximationsoftheapproach,inparticularthereexpansionof productsofSTOsintosingleSTOsandthenumericalquadratureisnotpresently known.
FortheothercurrentmolecularuseofSTObasissets,high-accuracydiatomic calculations,basissetshavebeendevelopedforselectedatomsinconnectionwith calculations.LesiukandMoszynskihavedevelopedhierarchiesofSTObasissetsfor Be[33],Ca,Sr,andBa[34].AstherearenogeneralbenchmarksavailableforhighaccuracycalculationsondiatomicmoleculesusingSTObasissets,thediscussionof theperformancemustnecessarilyberestrictedtothesecasestudies,andwewillhere focusonthecalculationontheBedimer.FortheBeatom,ahierarchyofbasissetswas developedforcardinalnumbersupto6usingtheideasofthecorrelation-consistent GTObasissets.Fornumericalstabilityinthebasissetoptimization,itwasfoundmost convenienttouseSTObasiswith n = + 1inEq.(1.2)inagreementwiththechoice forGTObasissets.Theexponentsoftheprimitivefunctionswerenotindividually optimized,butweredefinedinthreeorfourparametersandtheseparameterswere thenoptimized.TheuseofthesebasissetsfortheBedimerinconnectionwithbasis setextrapolationintheformofEq.(1.11)wasfoundtoprovideexcellentresults forthecompletebasissetlimit.TheuseofSTObasissetscouldthusexploitthe moreinvolvedthree-parameterextrapolation,andinparticularthereliabilityofthe extrapolationofthecontributionsfromthecorrelationofthe1s-orbitalwerefound togivebetterresultsthanforstandardGTOcorrelationconsistentbasissets.The bindingenergyofBedimerwastheoreticallydeterminedas929.0cm 1 withan estimatedaccuracyof1.9cm 1 ,whichconstitutesthebestcomputationalestimate ofthisenergy,bothintermsoftheagreementwiththeexperimentalvalueof929.7 ± 2.0cm 1 andintermsoftheestimatederrorofthecalculation.
TakingtheBedimerasaprototypicalexampleoftheaccuracythatmaybeobtained forsmalldiatomicmoleculesusingSTObasissets,itisconcludedthatbasissets basedonSTOsprovidesthehighestaccuracyatthemoment.Althoughtheevaluation oftheintegralsovertheSTOisaboutanorderofmagnitudelargerthanthatforGTO
basissetsofsimilarsize,theintegralevaluationtimeconstitutesonlyasmallfraction ofthetotaltime.ItispossiblethatGTObasissetswithsimilaraccuracymaybe developed,butthesearepresentlynotaround.
ManydevelopmentsarestillrequiredbeforeSTObasissetsmaybecomea choiceonparorevensupersedingGTObasisforgeneralcalculationsonpolyatomicmolecules.Manyofthedevelopmentsthathaveallowedquantumchemistry tobecomeageneralusefultoolfordescribingthestructure,energy,andspectraare stillmissing.Forexample,thedeterminationofequilibriumstructuresformolecules requiresthecalculationofderivativesofintegralswithrespecttothepositionof nuclearcenters.WhilethedevelopmentofsuchintegralsforGTObasissetswas completedabout30yearsago,thecorrespondingdevelopmentforSTObasissets hasnotyetbeeninitiated.Withoutafundamentalchangeinthewaythatintegralsover STOsareprogrammedandcalculated,itisnotlikelythattheuseofSTOwilltake overtheroleofGTObasissetsasageneraltoolforquantumchemicalcalculations, buttheymaybecomeacomplementforhigh-accuracycalculationsofenergeticsat givengeometries.
1.6BasisSetsWithLocalSupport,FiniteDifference, andSplineApproaches
Thevariousnumericalmethodsthatexploitbasisfunctionswithlocalsupportor spatialgridsnotonlyhavemanycommonfeatures,buthavealsoanumberofimportantdifferences.Letusforsimplicityrestrictourdiscussiontoasingledimension, wheretheseapproachesstartbyintroducingasetofpoints, r i ,spanningtheinterval ofinterest.Inthefiniteelement[40]andwavelet[41]approaches,thepointsare collectedintogroupsof k pointswiththefinalpointofonecellalsoincludedasthe firstpointofthefollowingcell.Polynomialsoforder k 1arethenintroducedin eachofthesecells,anditisthesepolynomialsthatformthebasisfunctionsofthe calculation.TheLegendrepolynomials[42]aresimpleandusefulexamplesofsuch localpolynomials.Inquantumchemistrycodes,polynomialsoforderupto8are oftenused.Thepolynomialsarechosensothattheexpansionofthebasisfunctions iscontinuousanditisalsopossibletoensurecontinuityofthefirstorhigherderivatives.FortheB-splineapproach[43],thepointsarenotgroupedintocells,andthe definedlocalpolynomialsspantheintervalfromagivenpoint r i andafewofthe followingpointswiththepolynomialschosensoatleastthefunctionandthefirst derivativesarecontinuous.
Expansionsoforbitalsintermsofthebasisfunctionsoflocalpolynomialsare inprinciplecompletelyequivalenttotheexpansionoforbitalsinthestandardbasis functions,buttherearetwoaspectsthatgivetheirquantumchemicalimplementation adifferentflavour:theverylargenumberofbasisfunctions,whichmayruninto hundredsofthousandsormillions,andthelocalityofthebasisfunctions,which greatlysimplifiesthecalculationofintegralsbetweenthese.
Restrictingthediscussiontogeneralpolyatomicmolecules,theextensivelyused programOctupus[44]isbasedontheFEMmethod.Thewaveletswereintroducedin quantumchemistrybyHarrisonandcoworkers[45],andhaverecentlybeenapplied togeneralmoleculesattheHFandDFTlevelsbyFredianiandcoworkers[46],where theyhavebeenusedtodeterminethefullbasissetlimitofenergiesandmagnetic properties.
PredatingtheFEMandwaveletapproachesisthestandardfinitedifference(FD) approach[47]ofnumericalanalysis,whichwasusedtocalculateapproximate (Hartree)wavefunctionsalreadyin1927[48],oneyearafterthepapersintroducing anddefiningquantummechanicswerepublished.TheFDapproachforatomshas beenextensivelydevelopedbyFroeseFischer[49]andcoworkers.Therehavealso beenseveraldevelopmentstoextendtheuseofFDmethodstomoleculesandsolids. Thesedevelopmentswerestartedbythetwo-dimensionalFDcodefordiatomic moleculesdevelopedbyLaaksonen,Sundholm,andPykko[50],andhavemost recentlybeenusedintheboxandbobleapproachofSundholmandcoworkers[51], whereideasofFEMandFDapproachesarecombined.Anumberofrealspacemethodsforsolidshavealsobeendevelopedinthelastdecadesandareinmanycontexts givingmorepredictableaccuracythanofferedbytheplanewaveexpansions.One ofthemostprominentandsuccessfuldevelopmentsalongtheselinesistheGPAW codeofThygesenetal.[52].
ThereisasubtledifferencebetweenthewaytheFEMandFDequationsare obtained.Consider,forexample,thederivationoftheHartree–Fockequations.Inthe FDapproach,onefirstobtainsageneralexpressionfortheHartree–Fockequation usingaformallycompleteexpansionoftheorbitals,andonesubsequentlydiscretizes thisequationonagrid.IntheFEMmethod,onestartsbyintroducingtheexpansion oftheorbitalsinthefiniteelementsintheenergyexpansion,andonethenderivesthe variationalcondition.Thus,intheFDmethod,oneperformsthediscretizationafter havingderivedthevariationalcondition,whereasintheFEMmethod,thevariational conditionisderivedforadiscretizedwavefunction.ThismakestheFEMexpression somewhatmorecomplicatedtoderiveandprogram,butensuresthattheprocedure isvariational.InaFEMHartree–Fockcalculation,thefinaloptimizedenergyisthus thelowestenergythatmaybeobtainedwiththegivensetofbasisfunctions,whereas thisdoesnotholdfortheFDapproach.WhetherthisvariationalpropertyoftheFEM approachissufficienttojustifytheaddedcomplexitycomparedtotheFDapproach dependsonthecomplexityoftheproblem.ForthestandardHFwavefunctions,where thevariationalproblemingeneralisrathersimple,theaddedcomplexityoftheFEM approachisingeneralnotwarranted,butforthemuchmorecomplexoptimization ofmulti-configurationalHartree–Fockwavefunctions,thevariationalaspectsofthe FEMapproachcertainlyhavetheiradvantages.
Theabovediscussionwasrestrictedtothecalculationofboundelectronicstates. Animportantadditionaluseofquantummechanicalmethodsisthecalculationof cross-sectionsforscattering,electrondetachment,andionization.Suchcalculations areimportantforproblemsrangingfromthereentranceofspacevehiclesinthe atmospheretothebehaviorofatomsandmoleculesinintenselaserfields.Thewave functionofanelectronbeingionizedfromamoleculecannotbeaccuratelydescribed
intermsofGTOsorSTOs,sobasissetswithlocalsupport,FEMpolynomialor splines,areusedtodescribethese.Thepointsofexpansion,thegridpoints,are typicalchosensothematrixelementsofapotentialoperatorbecomediagonalinthis basis.Thisisthefundamentalideaofthediscretevariablerepresentation(DVR)of FEM[53].Inpractice,amoleculeisdividedintotwoparts,aninnerpartwherethe standard(GTO)basissetisusedtodescribetheboundelectrons,andanouterpart wheretheoutgoingelectronisdescribedusingtheFEMbasisfunctionsorsplines.
1.7ConclusionandOutlook
Althoughsignificantdevelopmentsarebeingmadewithineachoftheabovementionedformsofbasissets,themostinnovativedevelopmentswithinrecentyears havearguablybeenthecombineduseofseveraldifferentformsofbasissets.One maytherebyusethevariousformsofbasissetexpansions,wheretheyhavetheir strengths.Oneofthecurrentlymostprominentofsuchapproachesisthecombined useofaGTOandaplanewavebasistodescribesolids.TheGTObasisisusedto describethecoreandinnervalenceelectrons,astheirhighlylocalizedformmakes themdifficulttodescribeusingplanewaves,whereastheplanewavesareusedto describetheouterdelocalizedandmuchslowervaryingpartofthewavefunction. TheabovementioneduseofcombinedGTOandFEMexpansionsforscatteringis anotherexampleofsuchacombination.Itisthefirmopinionofthepresentauthor thatmuchprogressisstilltobeobtainedbysuchcombinationsofdifferentforms ofbasissets.Itisthehopethatthepresentbookmayinspireandfacilitatesuch advances.
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Chapter2 Slater-TypeOrbitals
DevashisMajumdar,PabitraNarayanSamanta,SzczepanRoszak, andJerzyLeszczynski
Abbreviations
ADFAmsterdamdensityfunctionalcode a0 Bohrradius
ALDAAdiabaticlocaldensityapproximation CCSDCoupledclustersinglesanddoubles CCSD(T)CCSDincludingtripleexcitations
CGTOContractedGTO CTChargetransfer DFTDensityfunctionaltheory EOMEquationofmotion F Fockmatrixoperator
GTOGaussian-typeorbital h One-electronoperator HOMOHighestoccupiedmolecularorbital J Coulomboperator
K Exchangeoperator
D.Majumdar(B) P.N.Samanta J.Leszczynski(B) InterdisciplinaryCenterforNanotoxicity,DepartmentofChemistry,PhysicsandAtmospheric Sciences,JacksonStateUniversity,Jackson,MS39217,USA e-mail: devashis@icnanotox.org
J.Leszczynski e-mail: jerzy@icnanotox.org
P.N.Samanta
e-mail: pabitra.samanta@icnanotox.org
S.Roszak(B) AdvancedMaterialsEngineeringandModellingGroup,FacultyofChemistry,Wroclaw UniversityofScienceandTechnology,Wyb.Wyspianskiego27,50-370Wroclaw,Poland e-mail: szczepan.roszak@pwr.wroc.pl
©SpringerNatureSwitzerlandAG2021
E.Perlt(ed.), BasisSetsinComputationalChemistry,LectureNotesinChemistry107, https://doi.org/10.1007/978-3-030-67262-1_2
18D.Majumdaretal.
LDALocaldensityapproximation
LCAOLinearcombinationofatomicorbital
LUMOLowestunoccupiedmolecularorbital
M-LMetal–ligand
MOMolecularorbital
NLNonlocal
PAPolyacetylene
RIRamanintensity
SCFSelf-consistentfield
SERSSurface-enhancedRamanspectroscopy
SOCSpin–orbitcoupling
STOSlater-typeorbital
TDDFTTime-dependentdensityfunctionaltheory
VKVignale–Kohn
VWNVosko–Wilk–Nausair
XCExchange–correlation
ZORAZeroth-orderregularapproximation
ζ Slaterexponent
2.1Introduction
MolecularorbitalsinquantumchemistryarisefromthesolutionofHartree–Fock (HF)-typeequationsandtheyareusedtocomputevariouselectronicproperties relatedtothemolecularstructure[1].Afewofsuchpropertiesaredipolemoment, chargedensityonatoms,Badercharacteristics,excited-stateproperties(which includetransitionmoment,oscillatorstrengthoftheexcitedstates,etc.),vibrational andRamanspectraofmolecules,andsoon.Thesemolecularpropertycalculations arenotsimplyrestrictedtosimpleHFequations.Theyalsoincludedensityfunctionaltheory(DFT)calculations[2],coupledcluster(CCSD,CCSD(T),etc.)[3–7], andmanyothersingleandmultireferencetechniquesatvariouslevelsofsophistication[1, 8–10].Forthepresentpurpose,itiseasiertoconsiderjustHFequations tointroducetheconceptofmolecularorbitals.Ifweconsideran n-electronclosedshellmolecule,theground-statewavefunction couldbewrittenasasingleSlater determinantformas:
Themolecularorbitals ψ i oftheitheigenstate ε i ariseasasolutiontotheHF equation F ψ i = ε i ψ i (F:Fockmatrixoperator)whichisanintegro-differential equation.The F isusuallywrittenasacombinationofone-electronoperator h and two-electronoperators J and K as[8],
Thecoulomboperator J (Eq. 2.4)representstheaveragelocalpotentialat r 1 arisingfromanelectronin ψ j (at r 2 ),whiletheexchangeoperatorKin(2.5)is definedbyitseffectwhenoperatingonspinorbital ψ i (r1 ).Theexchangeintegrals computedthrough(2.5)as K ψ i (r1 )shouldvanishunlessthespinfunctionof ψ j (r 2 ) isthesameas ψ i (r1 ),whilethecoulombintegrals J ψ i (r1 )arenon-vanishing.The integro-differentialnatureoftheHFequationisalsorevealedthroughoperators h (thekineticenergypartrevealingthedifferentialcomponent), J ,and K (providing anintegralpart).Thesolutionsofsuchequationsareusuallyachieved variationally withastartingguessfor ψ i .ThesolutionsofHFequationsareonlyfeasible foratomsandlinearmolecules,wheretheangularpartof ψ i couldbeprecisely separatedfromtheradialpartduetothepresenceofaxialandfullrotationalgroup symmetry.Ontheotherhand,inthecaseofanonlinearmolecule,theHFequations cannotbesolvedbyfollowingasimilarmathematicalapproachowingtotheirthreedimensionallandscapeof ψ i aswellasthepotentialenergytermsthatappearedin F.
Themostcommonlyusedtechniqueistoexpand ψ i intermsofbasisfunction ϕ μ accordingtotheLCAO-MO(LCAO:linearcombinationofatomicorbital;MO: molecularorbital)procedure:
ThisreducesHFequationtotheform:
Equation(2.7)isgenerallycalledtheRoothaan–Hallequation.Itintroducesthe conceptofatomicbasisfunctions(orbasissets)tosolvetheHFequations variationally.The Sμν termistheoverlapmatrixelementbetweentheatomicorbitals(AOs) μ andv, F μν istheFockmatrixelementand c ν i istheLCAO-MOcoefficient.Itshould benotedthatthematrix F μν hasadimensionNequaltothenumberofatomicbasis
20D.Majumdaretal.
orbitalsusedintheLCAO-MOexpansion,andhasNeigenvalues ε i andNeigenvectorswhoseelementsare c ν i .Thus,thereareoccupiedandvirtualmolecularorbitals (MOs),whicharedescribedby c ν i coefficientsthroughthesolutionofEq.(2.7).
2.2ConceptofBasisSets
Asaconsequenceofatomicsymmetry,atomicorbitalsarealwaysoftheform[1, 8]:
where R(r) istheradialpartand Y lm (θ ,ϕ ) representssphericalharmonics.Duetothe singularityofthepotentialatapointnucleuswithachargeofZ,thewavefunction musthaveacuspatthenucleus.Morespecifically,itisrequiredthat
Attheotherendoftherange,anelectronfarawayfromanymoleculewould seetheremainderofthemoleculeasapositivechargewithoutanyparticularstructure.Therefore,thewavefunction,likeinanysingle-electronatom(cf .thecaseof H-atom),woulddecayexponentially.Theconsiderationofsuchideasledtothe developmentofatomicbasissets.Thebasisorbitals(orbasissets)commonlyused inLCAO-MO-SCF(SCF:self-consistentfield)calculationsfallintotwoclasses,viz. Slater-typeorbitals(STOs)andGaussian-typeorbitals(GTOs).TheSTOscouldbe mathematicallyrepresentedas:
TheseSTOsarecharacterizedbythequantumnumbers n, l,and m (principal, azimuthal,andmagnetic,respectively)andtheexponent ζ (characterizestheradial size ofthebasisfunction).
GTOsareusuallywrittenintermsofquantumnumbers a, b,and c andCartesian coordinates x, y,and z as:
IntheCartesianrepresentationofGTOsinEq.(2.11),thequantumnumbers a,b, andc representsdifferentatomicorbitals.Forexample,orbitalswith a,b,c values of1,0,0,or0,1,0or0,0,1are px , py ,and pz orbitals;thecombinations2,0,0,or0,2,0, or0,0,2and1,1,0,or0,1,1,or1,0,1areforfive d orbitalandone s orbital(thesum ofthe2,0,0,0,2,0,and0,0,2orbitalsisansorbitalbecause x 2 + y2 + z 2 = r 2 is independentof θ and ϕ ).Thus,thequantumnumbers a,b, andc describetheangular
shapeanddirectionoftheorbital,whiletheexponent α governstheradialsizeof thebasisfunction.ForbothSTOsandGTOs,thevaluesof r , θ , and ϕ signifythe localizationofelectronwithrespecttoanarrayofaxesassignedtothecenteron whichthebasisorbitalissited[8].
The2p,3d, etc.,intheSTOsandGTOsaregeneralizationsofEqs.(2.10)and (2.11)thathaveapolynomialinthecomponents r (x, y,etc.)multiplyingthesame exponential e ζ r orGaussian e α r 2 fall-off.Theorbitalexponents,whichare positivenumberslargerthanzero,determinethediffusenessor size ofthebasis function.Alargeexponentimpliesasmalldensefunctionandasmallexponent impliesalargediffusefunction.Themajordifferencesbetweenthetwofunctions e ζ r and e α r 2 occurat r = 0andatlarge r .At r = 0,theSlaterfunctionhasafinite slopeandtheGaussianfunctionhasazeroslope[1],
Atlargevaluesof r , e α r 2 decaysmuchmorerapidlythantheSlaterfunction e ζ r .
AllSTOsmorecorrectlydescribethequantitativefeaturesofthemolecularorbital ψ i thandoGTOs,andfewerSTOswouldbeneededinbasisfunctionexpansionof ψ i forcomparableresults.Asanexample,itispossibletoshowthatatlargerdistances, molecularorbitaldecaysas ψ i → exp (-ai r ),whichisoftheSlaterform.Forthe sakeofsimplicity,themolecularorbitalsofhydrogenatomhavebeentakeninto consideration.The 1s, 2s,and 2px orbitalscouldberepresentedas[8]:
(r :radialcoordinateoftheelectron, a0 :Bohrradius;nuclearcharge = 1)Consideringsuchproperties,STOscouldbejudgedaspreferablethanGTOsinelectronic wavefunctioncalculations.ThestumblingblockforusingSTOsappearsduringthe calculationsofmulticentermulti-electron J and K integralsdefinedinEqs.(2.4) and(2.5).TheseintegralscouldberoutinelycalculatedinGTOs,astheyhavethe advantageofexpressingthesemulticenterintegralsintermsofthesamecenter.For example,thetwo-centerproductsinGTOscouldbeexpressedas[1, 8]:
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and arouses religious feeling. Oratorio was his special gift to the world and one never hears the name of Handel without thinking of The Messiah.
Handel seemed to reunite the forms: oratorio and opera, under his massive will. At first some of his oratorios were given in costume, showing the influence of opera.
Handel had many enemies in England, but he also had friends. Although imperious, he had a sweet side, and made friends with humble folk who loved music, even though he hobnobbed with royalty. Thomas Britton, a coal heaver, his friend, is sketched by an artist of the day in a picture where Handel is playing The Harmonious Blacksmith to Alexander Pope, the Duchess of Queensbury, Colley Cibber and other famous folk. Yet he stormed at everyone and even royalty “quaked in their boots” and were forced to behave themselves at rehearsals and concerts which Handel directed.
Accused of using someone’s melody, he answered, “That pig couldn’t use such a melody as well as I could!” He helped himself to so many that he was called the “Great Plagiarist.”
His latter life was spent quietly, with a few intimate friends, drinking his beer and smoking his beloved pipe. He was always generous and as he grew older seemed to become kindlier and softer. He contributed largely to the Foundling Asylum and even played the organ there.
He wanted to die on Good Friday, “in hopes,” he said, “of meeting his good God, his sweet Lord and Saviour on the day of his resurrection,” and on Good Friday, April 6th, 1759, he died.
C W