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1、AnIntroductiontoBasisSets:The"HolyGrail"ofcomputationalchemistryisthecalculationofthemolecularorbitals(MOs)foragivenmolecule.IFwecancalculatetheMOsforamolecule,THENwecanknowlotsofthingsaboutthemolecule,includingits:•energy•electrondensity•electrostaticpotential•tr
2、ansitionstate(ifany)•frequencyHowever,calculatingMOsisnotthateasy.Thecomputer,however,doesthisforus.But,you,thechemist,musttellthecomputersomeinformationfirst.Inputfilesforallofthemajorcomputationalchemistrypackagescontainthesethreeparts:1.GeometrywhichincludesoBo
3、ndlengthsoBondAnglesoDihedrals2.KindofCalculations:oSinglepointenergyoFrequencyoTransitionstateoElectrondensityoElectrostaticpotential3.StartingsetofmathematicsandapproximationsoCalculationmethod(molecularmechanics,semiempirical,orabinitiooType(s)ofapproximation(H
4、artree-Fock,Moller-Plesset,etc.)oBasisSetApproximationThegraphicbelow"captures"theessenceofwhatistheresponsibilityofthechemistandwhatistheresponsibilityofthecomputer:Inthislabwewillbelookingatthethirdingredient,basissets.Amolecular-orbitaltheorycalculationisamathe
5、maticalexpressionofanelectroninamolecule.Althoughtherearemanytypesofmolecular-orbitalfunctions,inthislabwewillonlylookattheSlaterTypeOrbitals(STOs)andtheGaussianTypeOrbitals(GTOs).Althoughthereisnotamajordifferenceinthesetwomethodswhencalculatingsmallmolecules,maj
6、ordiscrepenciesariseforlargermoleculesof30ormoreatoms.STOsrequiremorecalculating,whichtakestremendousamountsoftime,howevertheircalculationshavebeenfoundtobemoreaccuratethanGTOs.Ontheotherhand,GTOs,althoughlessaccurate,aremuchfastertocalculatethanSTOs.Thisforcedsci
7、entiststocompromisetimeoraccuracy.Eventually,scientistsrealizedthatbyaddingseveralGTOs,theywereabletomimictheSTOsaccuracy.Infact,asthenumberofGTOsusedincreased,thebettertheywereabletomodeltheSTOequation.WhenusingGTOstomodelSTOs,thenewequationsaregivenanewname.They
8、areidentifiedasSTO-kGequationswherekisaconstantthatrepresentsthenumberofGTOsused.Forinstance,twocommonequationsaretheSTO-3GandtheSTO-6Ginwhich3and6GTOsa
