General guidelines for BOVB calculations

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Active space

First, choose an active system of orbitals/electrons in the molecule under study. This is the part of the molecule that will be treated in a VB way : for example the bonds that are made/broken in a reaction, or the orbitals whose occupancy varies in a series of resonating VB structures. The occupancies of the active orbitals will change from one VB structure to the other. The other orbitals, called « inactive » or « spectator », are those that keep the same occupancy in all VB structures.

BOVB levels

Each active orbital must be defined as localized, either on a single atom or on a fragment (recommended, see next section below). The inactive orbitals may be kept localized (L-VBSCF or L-BOVB) or allowed to delocalize on the whole molecule (D-VBSCF or D-BOVB). Better accuracy is obtained in this latter case. Last, in the ionic structures the doubly occupied active orbitals may be "splitted" for a better description, leading to the S-BOVB (if inactive are kept localized) and SD-BOVB levels.

Recommended definition for the orbital blocks

These rules lead to the definition of "orbital blocks" :

  • From your active orbitals divide the molecule into fragments, such as two active orbitals involved in a covalent coupling in one of the structures belong to different fragments;
  • To each fragment is associated a localization space : the orbitals belonging to a specific localization space can only span the basis functions centered on atoms belonging to this fragment;
  • The different localization spaces may then be further divided according to the symmetry of the molecule (<math>\sigma</math>/<math>\pi</math> separation for instance), which provides the different orbital blocks.
BOVB-blocks.png
Two example choices of orbital block for Cl-(Me3)C-Cl- SN2 transition state. Note that the second choice corresponds to the so-called π-D-VBSCF and π-D-BOVB levels, and that they won't be valid for reactant and product geometries where there is no formal <math>\sigma</math> / <math>\pi</math> separation.

Procedure

To perform a D-BOVB calculation :

  1. Perform a ("L") VBSCF calculation together with the keyword boys in the $orb section ;
  2. perform the L-BOVB calculation, always starting from converged VBSCF orbitals ;
  3. then perform the D-BOVB calculation : starting from the converged L-BOVB orbital as guess, run the calculation by optimizing the delocalized inactive orbitals, while freezing the active orbitals during the optimization (put "0" as coefficient in first $orb line).

>> SD-BOVB calculations (advanced user)

General advices

  • Do not use diffuse functions unless you deal with an anion, and do not use larger than triple-zeta basis sets
  • Impose a high molecular symmetry if possible (case of a distorted molecule which slightly departs from a higher symmetry point group) ;
  • Use orbtyp=hao together with fragtyp=sao as soon as you have some <math>\sigma</math>/<math>\pi</math> symmetry in your molecule,
  • In the $ctrl section of the XMVB input, you should use the "iscf=5" algorithm for VBSCF calculations, and change it to "iscf=2" for BOVB calculations.
  • Use the boys keyword at the VBSCF step, as it provides more physically meaningful orbitals for the subsequent BOVB calculations (this is particularly important if you want to go up to the S- or SD-BOVB levels)
  • Always use a set of converged orbitals from the preceding step, ex : use converged VBSCF orbitals to start a L-BOVB calculation, converged L-BOVB to start a S-BOVB or a D-BOVB calculation, and converged S-BOVB orbitals to start a SD-BOVB calculation.