量子化學(xué)教學(xué)課件蘇州大學(xué)第八章密度泛函理論簡(jiǎn)介課件

?量子化學(xué)?樊建芬Chapter8IntroductionofDensityFunctionTheory第八章密度泛函理論簡(jiǎn)介1從20世紀(jì)60年代密度泛函理論〔DFT〕提出以來(lái)，并在局域密度近似下導(dǎo)出著名的Kohn-Sham方程以來(lái)，DFT已逐漸成為量子化學(xué)計(jì)算領(lǐng)域的強(qiáng)有力的工具。在量子化學(xué)計(jì)算領(lǐng)域，據(jù)INSPEC數(shù)據(jù)記錄顯示:①直至80年代末，分子軌道HF方法一直占主導(dǎo)地位；②90年代中期，DFT和HF方法的論文并駕齊驅(qū)；之后，DFT的工作按指數(shù)級(jí)數(shù)增加。2最近幾年，密度泛函方法得到了廣泛的應(yīng)用，通?？梢缘玫奖菻F方法更好的計(jì)算結(jié)果，但只需中等程度的價(jià)格，對(duì)于中型或大型分子體系，其計(jì)算本錢遠(yuǎn)低于MP2法。

1998年，Kohn因DFT的開創(chuàng)性工作與Pople共同獲得諾貝爾化學(xué)獎(jiǎng)。從中可見DFT在化學(xué)領(lǐng)域中的地位。3電子動(dòng)能算符核－電子吸引能電子－電子排斥能電子相關(guān)交換能+++[]

=E

注：①電子相關(guān)能：主要包括自旋━軌道偶合作用能、自旋━自旋偶合作用能、軌道━軌道偶合作用能。②電子交換能，電子具有全同粒子特性，滿足保里原理，波函數(shù)必須具有反對(duì)稱性，它與非對(duì)稱波函數(shù)的能量差為交換能。分子體系的Schr?dinger方程234兩種狀態(tài)的波函數(shù)不同，能量也有所差異。例：Li+的某一激發(fā)態(tài)1s12s1,假設(shè)電子排布狀態(tài)為1s2s為非對(duì)稱波函數(shù)為反對(duì)稱波函數(shù)5但是它在描述化學(xué)鍵時(shí)有些缺乏，而且沒有相關(guān)能校正時(shí)不宜處理熱化學(xué)問題。HF理論能夠在分子尺寸提供較準(zhǔn)確的交換能處理，同時(shí)也是大體系的實(shí)際計(jì)算工具。HF超自洽場(chǎng)的校正，如MP多級(jí)微擾、組態(tài)相互作用CI也只能處理中小分子，對(duì)大體系是不實(shí)用的。6Densityfunctionaltheory(DFT)attemptstocalculateandotherground-statemolecularpropertiesfromtheground-stateelectrondensity

Proof:TheelectronicHamiltonianisexternalpotential11目錄9InDFT,iscalledtheexternalpotentialactingonelectroni,sinceitisproducedbychargesexternaltothesystemofelectrons.Oncetheexternalpotentialandthenumberofelectronsnarespecified,theelectronicwavefunctionsandallowedenergiesofthemoleculearedeterminedasthesolutionsoftheelectronicSchr?dingerequation.

10(1)theexternalpotential(exceptforanarbitraryadditiveconstant);Itcanbeprovedthattheground-stateelectronprobabilitydensitydetermines:(2)thenumberofelectrons.911“Forsystemswithanondegenerategroundstate,theground-stateelectronprobabilitydensitydeterminestheground-statewavefunctionandenergy,andotherproperties”。

emphasizesthedependenceofontheexternalpotential,whichdiffersfordifferentmolecules.

12Here,However,thefunctionalsareunknown.

13isalsowrittenasHere,Thefunctionalisindependentoftheexternalpotential.

目錄

148.2TheHohenberg-kohnvariationaltheoremthefollowinginequalityholds:whereisthetrueground–stateenergy.”“Foreverytrialdensityfunctionthatsatisfiesandforall,目錄15Letsatisfythatand.BytheHohenberg-Kohntheorem,determinestheexternalpotentialandthisinturndeterminesthewavefunctionthatcorrespondstothedensity.Proof:

16LetususethewavefunctionasatrialvariationfunctionforthemoleculewithHamiltonian.Accordingtothevariationtheorem

17SincethelefthandsideofthisinequalitycanberewrittenasHohenbergandKohnprovedtheirtheoremsonlyfornondegenerategroundstates.Subsequently,Levyprovedthetheoremsfordegenerategroundstates.

Onegets

目錄

188.3TheKohn-ShammethodIfweknowtheground-stateelectrondensity,theHohenberg-Kohntheoremtellsusthatitispossibleinprincipletocalculatealltheground-statemolecularpropertiesfrom,withouthavingtofindthemolecularwavefunction.419

Theirmethodiscapable,inprinciple,ofyieldingexactresults,butbecausetheequationsoftheKohn-Sham(KS)methodcontainanunknownfunctionalthatmustbeapproximated,theKSformationofDFTyieldapproximate

results.1965,KohnandShamdevisedapracticalmethodforfindingandforfindingfrom.[Phys.Rev.,140,A1133(1965)].

20.KohnandShamconsideredafictitiousreferencesystemsofnnoninteractingelectronsthateachexperiencethesameexternalpotentialthatmakestheground-stateelectronprobabilitydensityofthereferencesystemequaltotheexactofthemoleculeweareinterestedin:

21

Sincetheelectronsdonotinteractwithoneanotherinthereferencesystem,theHamiltonianofthereferencesystemiswhereistheone-electronKohn-ShamHamiltonian.

22TheKSoperatoristhesameastheHFoperatorexceptthattheexchangeoperatorsintheHFoperatorarereplacedby,whichhandlestheeffectsofbothexchangeandelectroncorrelation.

23VariousapproximatefunctionalsareusedinmolecularDFTcalculations.Thefunctionaliswrittenasthesumofanexchange-energyfunctionalandacorrelation-energyfunctional:4

24Amongvariousapproximations,gradient-correctedexchangeandcorrelationenergyfunctionalsarethemostaccurate.Commonlyusedand.

25Lee-Yang-Parr(LYP)functionalP86(thePerdew1986correlationfunctional)PW91(PerdewandWang’s1991functional)PW86(PerdewandWang’s1986functional)B88(Becke’s1988functional)目錄268.4CommentsontheDFTmethods(1)TheKSequationsaresolvedinaself-consistentfashion,liketheHFequations.(2)ThecomputationtimerequiredforaDFTcal