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Basics of VB theory and XMVB program

Main exercises

Exercise 1 : Starting up with the H<math>{}_2</math> molecule The Gamess and XMVB input files for the H<math>{}_2</math> molecule are provided in the Exercise folder on the tutorial machines. These are VBSCF calculations with the 6-31G(d,p) basis set, and the fragment specification in terms of symmetry-adapted orbitals (frgtyp=sao). Just inspect these inputs, run the gamess-xmvb program (using : vbrun h2), and analyze the outputs. Then these input files could serve you as templates for the next exercises. Exercise 2 : HF molecule weights Compute a VBSCF three structure wave function for the HF molecule, using the frgtyp=sao specification, automatic guess (guess=auto), and boys keyword in the $tctrl section. Which structure(s) should be kept in further BOVB calculations ? Using VBSCF orbitals as guess orbitals : Compute a L-BOVB wave function on a selected subset of structures ; Compute a VBCISD wave function, freezing the 1s core orbital of fluorine in the VBCI calculation (NCOR=1 option), and printing only structures which have a coefficient superior to 0.01 (ctol=0.01 option) ; Compare structure weights at the VBSCF, L-BOVB and VBCI levels Hints To go from L-VBSCF to L-BOVB level, starting from the input of the VBSCF input file as a template, you should simply make the following changes : add the bovb keyword in "$ctrl" section ; change iscf=5 by iscf=2 ; suppress structures with minor weights at the VBSCF level from the '$str section use "guess=read" option and previous converged VBSCF orbitals as input.gus guess file To go from L-VBSCF to the VBCI wave functions, starting from the input of the VBSCF input file as a template, you should simply make the following changes : add the corresponding VBCI keyword in the $ctrl section (VBCISD here) ; use "guess=read" option and previous converged VBSCF orbitals as input.gus guess file >> general guidelines for BOVB calculations Exercise 3 : F<math>{}_2</math> molecule and bond energy Compute a L-VBSCF wave function for the F<math>{}_2</math> molecule (all inactive orbitals localized on the fluorine atoms), using: the cc-pvtz basis set ; the frgtyp=sao specification without the f basis functions (for simplicity) ; the boys keyword in the $ctrl section ; specifying a guess read from Gamess RHF Molecular orbitals (guess=mo in $ctrl section together with extra $gus section) ; BOVB level : First, compute a π-D-VBSCF wave function using previous VBSCF orbitals as guess orbitals. To do that, you should allow the π inactive orbitals of fluorine to delocalize onto the two atoms, while keeping all <math>/sigma</math> (active and inactive) orbitals localized (see also : >> see "high symmetry case" in the "general guidelines for BOVB calculations") Compute then a π-D-BOVB solution for the F<math>{}_2</math> molecule, starting from previous orbitals as guess. Compute a VBCISD solution starting from π-D-VBSCF orbitals (freezing the 1s core orbital of Fluorines in the VBCI calculation). Compute F<math>{}_2</math> bond energies at the π-D-BOVB and VBCI levels To compute the bond energy at the BOVB level, you can simply use the ROHF energies computed with Gamess for the separate H and F atoms, as the L-BOVB wave function dissociate to uncorrelated H+F fragments. To compute the bond energy at the VBCISD level, you should however compute the separate fragments at this level of theory. Hints To prepare a $gus section for reading RHF MOs as a guess : first compute gamess RHF solution only (take out : vbtyp=xmvb in the $control section of Gamess input) read the RHF orbitals in Gamess and identify those who could be good guess orbitals for : 1s core of F, 2s lone pair, 2px lone pairs,... active orbitals then build the $gus section in XMVB input accordingly, and start your calculation (don't forget to add again vbtyp=xmvb in the $control section of Gamess input) For the VBCISD calculation on F2 you should add NCOR=2 in the $ctrl section, as there is two core orbitals (core of each fluorine) to freeze in the calculation To compute the bond energy at the BOVB level, you can simply use the ROHF energies computed with Gamess for the separate fragments (F atoms here), because the L- and D-BOVB wave functions (like the VBSCF one) dissociate to uncorrelated separate fragments. To compute the bond energy at the VBCISD level, you should however compute the separate fragments at this level of theory.

Optional exercises - homework

Exercise 4 : The lone pairs of H2O (for further reading, see S. Shaik and P.C. Hiberty, "The Chemist's Guide to VB theory", Wiley, Hoboken, New Jersey, 2008, pp. 107-109) This exercise aims at comparing two descriptions of the lone pairs of H<math>{}_2</math>O : (i) the MO description in terms of non-equivalent canonical MOs and (ii) the « rabbit-ear » VB description in terms of two equivalent hybrid orbitals. <math>\Psi_{\textrm{MO}}</math>   <math>\Psi_{\textrm{VB}}</math> Focusing on the lone pairs only, write the four-electron single-determinants <math>\Psi_{\textrm{MO}} </math> and <math>\Psi_{\textrm{VB}} </math> . Expand <math>\Psi_{\textrm{VB}} </math> into elementary determinants containing only <math>n</math> and <math>p</math> orbitals, eliminate determinants having two identical spinorbitals, and show the equivalence between <math>\Psi_{\textrm{VB}}</math> and <math>\Psi_{\textrm{MO}}</math>. We now remove one electron from H<math>{}_2</math>O. Write the two possible VB structures <math>\Phi_1</math> and <math>\Phi_2</math> in the VB framework. By convention, one may write the doubly occupied lone pair first, then the singly occupied one. The two ionized states are the symmetry-adapted combinations and . From the sign of the hamiltonian matrix element <math>\langle \Phi_1 \vert \hat{H} \vert \Phi_2 \rangle</math>, give the energy ordering of the two ionized states. By expanding the two ionized states into elementary determinants (dropping the normalization constants), show that they are equivalent, respectively, to the MO configurations <math>\vert nn\bar{p}\vert</math> and <math>\vert pp\bar{n}\vert</math>. Appendix Hamiltonian matrix element between determinants differing by one spin-orbital : <math>\langle \cdots i \cdots \vert \hat{H} \vert \cdots j \cdots\rangle = \beta_{ij}</math> Don’t forget to put the orbitals of the two determinants in maximal correspondence before applying the rule. Answer 1. <math> \Psi_{\textrm{MO}}=\vert n\bar{n}p\bar{p}\vert </math>   ;   <math> \Psi_{\textrm{VB}}=\vert \left( n-\lambda p\right)\left( \bar{n}-\lambda\bar{p}\right)\left( n+\lambda p\right)\left(\bar{n}+\lambda\bar{p}\right)\vert </math> 2. <math> \begin{matrix} \Psi_{\textrm{VB}} &=&\vert \left( n\bar{n} -\lambda p\bar{n} -\lambda n\bar{p} +\lambda^2 p\bar{p} \right) \left( n\bar{n} +\lambda p\bar{n} +\lambda n\bar{p} +\lambda^2 p\bar{p} \right) \vert \end{matrix} </math> <math> \begin{matrix} \Psi_{\textrm{VB}} &=&\vert n\bar{n}n\bar{n} \vert + \lambda\vert n\bar{n}p\bar{n} \vert + \lambda\vert n\bar{n}n\bar{p} \vert + \lambda^2 \vert n\bar{n}p\bar{p} \vert - \lambda \vert p\bar{n}n\bar{n} \vert -\lambda^2 \vert p\bar{n}p\bar{n} \vert -\lambda^2 \vert p\bar{n}n\bar{p} \vert -\lambda^3 \vert p\bar{n}p\bar{p} \vert \end{matrix} </math> <math> \begin{matrix} - \lambda \vert n\bar{p}n\bar{n} \vert -\lambda^2 \vert n\bar{p}p\bar{n} \vert -\lambda^2 \vert n\bar{p}n\bar{p} \vert -\lambda^3 \vert n\bar{p}p\bar{p} \vert + \lambda^2 \vert p\bar{p}n\bar{n} \vert +\lambda^3 \vert p\bar{p}p\bar{n} \vert +\lambda^3 \vert p\bar{p}n\bar{p} \vert +\lambda^4 \vert p\bar{p}p\bar{p} \vert \end{matrix} </math> After eliminating all determinants having two orbitals with the same spin, there remains : <math> \begin{matrix} \Psi_{\textrm{VB}} &=& \lambda^2 \vert n\bar{n}p\bar{p} \vert -\lambda^2 \vert p\bar{n}n\bar{p} \vert -\lambda^2 \vert n\bar{p}p\bar{n} \vert + \lambda^2 \vert p\bar{p}n\bar{n} \vert \end{matrix} </math> After permuting the columns and changing signs accordingly, there remains : <math> \Psi_{\textrm{VB}}=4\lambda^2\vert n\bar{n}p\bar{p} \vert=\Psi_{\textrm{MO}} </math> (if one includes normalization factors). 3. <math>\Phi_1=\vert \left( n-\lambda p \right)\left( \bar{n}-\lambda\bar{p} \right)\left( n+\lambda p \right) \vert </math>, <math>\Phi_2=\vert \left( n+\lambda p \right)\left( \bar{n}+\lambda\bar{p} \right)\left( n-\lambda p \right) \vert </math> Permuting the first and third orbitals in <math>\Phi_2</math> and changing the sign accordingly, we get <math>-\Phi_2</math> that has maximum orbital correspondence with <math>\Phi_1</math> : <math>-\Phi_2</math> =<math>\vert \left( n-\lambda p \right) \left( \bar{n}+\lambda\bar{p} \right) \left( n+\lambda p \right) \vert </math>. 4. <math>\Phi_1</math> and <math>-\Phi_2</math> differ by only one orbital, <math>\left( \bar{n}-\lambda \bar{p} \right)</math> in <math>\Phi_1</math> which becomes <math>\left( \bar{n}+\lambda \bar{p} \right)</math> in <math>-\Phi_2</math>. Therefore the matrix element <math> \langle \Phi_1 \vert \hat{H} \vert -\Phi_2 \rangle </math> is a simple <math>\beta</math> integral, necessarily negative. Hence, the lowest ionized state is <math> \frac{1}{\sqrt{2}}\left(\Phi_1-\Phi_2\right)</math> while the higher ionized state is <math> \frac{1}{\sqrt{2}}\left(\Phi_1+\Phi_2\right)</math>. 5. <math>\Phi_1=\vert n\bar{n}n\vert -\lambda\vert p\bar{n}n\vert -\lambda\vert n\bar{p}n\vert +\lambda^2\vert p\bar{p}n\vert+\lambda\vert n\bar{n}p\vert-\lambda^2\vert p\bar{n}p\vert-\lambda^2\vert n\bar{p}p\vert+\lambda^3\vert \bar{p}p\vert</math> <math>\Phi_1=+2\lambda^2\vert p\bar{p}n \vert +2\lambda\vert n\bar{n}p \vert</math> In the same way, one shows that <math>\Phi_2=-2\lambda^2\vert p\bar{p}n \vert +2\lambda\vert n\bar{n}p \vert</math>. It follows that : <math>\left(\Phi_1-\Phi_2\right)\propto \vert n\bar{n}p \vert </math> (lowest ionized state in MO theory) <math>\left(\Phi_1+\Phi_2\right)\propto \vert p\bar{p}n \vert </math> (higher ionized state in MO theory). It is concluded that 1) VB theory yields two ionization potentials for H<math>{}_2</math>O, in agreement with experiment, and 2) that these ionization potentials are exactly the same as the ones found in elementary MO theory. Exercice 5 : Solvent effect on C(Me)<math>{}_3</math>-Cl weights C(Me)<math>{}_3</math>-Cl at equilibrium geometry : Compute a VBSCF wave function using frgtyp=atom and a $Gus section to specify guess orbitals. The active electron pair will be the C-Cl bond, and all inactive orbitals should be localized either on Cl or on the C(Me3) fragment. Which structures should be kept in further BOVB calculations ? Redo the VBSCF calculation reading the orbital file obtained at the previous step as guess file, and now requesting a boys localization (keyword : boys). Compare the VBSCF orbitals obtained with and without the boys keyword (you can use the moldendat utility and them molden to display them). Compute a L-BOVB wave function. Compute a D-BOVB wave function, by freezing the active orbitals, and delocalizing all inactive orbitals onto the whole molecule (see also : >> general guidelines for BOVB calculations). Starting from guess orbitals obtained at equilibrium geometry, redo the D-BOVB calculation for the large inter fragment distance. How does the weights of the different structures evolve when the molecule is stretched ? Redo the D-BOVB calculations at equilibrium geometry and large distance using VBPCM for water. How does the weights change with solvation effects ? Hints and remarks It is strongly advisable to use the boys keyword at the VBSCF step, as it will provide more physically meaningful orbitals for the next BOVB calculations (the same remark would hold with VBCI) To reperform D-BOVB at large interatomic distances (question n°2), you have to proceed in two steps : starting from the orbitals converged at equilibrium distances (question n°1.3) as a guess : reperform a L-BOVB first at large interatomic distances ; then, starting from the orbitals just obtained as a guess do a D-BOVB calculation. Same remark for solvent calculations : you can start from converged gas phase L-BOVB orbitals as guess, but you'll have first to reperform L-BOVB and then D-BOVB.