General guidelines for BOVB calculations

General procedure for BOVB calculations

General case

Definition of the localization space

The following rules define the localization space of your orbitals for a "L" level (VBSCF or L-BOVB) calculation :

• First, choose a set of active electron pairs, which in turns define a set of active orbitals ;
• The inactive orbitals are initially delocalized on a fragment, and the molecule is fragmented in a way such that each fragment contain at most ONE atom bearing at least an active electron in at least one of the structures ;
• The active orbitals can be localized on the fragments (recommended), or on 1 atom only ;

Exemples :

1. for the (Me₃)C-Cl molecule, where we choose the C-Cl bond to be the active electron pair, the inactive orbitals are defined on the (Me₃)C and Cl fragments respectively
2. for Cl-(Me₃)C-Cl^- SN2 transition state, the two Cl-C and C-Cl bonds are chosen as active pairs, which in turns define three fragments : Cl1 / (Me₃)C / Cl2

Procedure for the calculation

• Perform a ("L") VBSCF solution ;
• L-BOVB, always starting from converged VBSCF orbitals ;
• D-BOVB : from a converged L-BOVB guess, freezing the active orbitals, and delocalizing the inactive orbitals onto the whole molecule

High symmetry case

Definition of the localization space

If the active and inactive orbitals do not share ANY basis function in common (like when there is a sigma/PI separation for instance, and all PI pairs are defined as active), then you can start with :

• inactive orbitals delocalized onto the whole molecule from the very beginning
• active orbitals localized on 1 atom only.

Procedure for the calculation

• Perform a D-VBSCF solution ;
• D-BOVB : from a converged D-VBSCF guess.

In case of trouble

How can I know if my BOVB calculation went well ?

Check the following quantities :

• the BOVB weights should not change dramatically as compared with VBSCF weights (not more than +/- ~5%) ;
• the overlap matrix between 2 given structures should not exceed ~0.7
• Coulson-Chrigwin weights : large negative weights (<-0.05) are a sign of convergence on a unphysical solution ;
• Look to the orbital overlap matrix (in the ".xdat" file) between active orbitals : corresponding active orbitals should have almost 1. overlap (0.98/0.99x). When it is not the case : inspect the corresponding orbital to check what it has become
• The total energy becomes very low, leading to, for instance, dissociation energies which might be (significantly) larger than exact ones ;

What can I do if I encounter a BOVB instabilities ?

• Check the following points :
  • Did I eliminate structures with minor (<1%) weight at VBSCF level from my BOVB calculation ?
  • Did I precisely follow the procedures described above to get my D-BOVB wave function ?
• How to cure the problem - if I haven't done any particular "mistake" :
  • modify slightly the definition of the wave-function (for instance : having active orbital strictly localized on 1 atom instead of fragment orbitals) ;
  • work in a smaller basis set (instabilities occur more often in extended basis sets) ;
  • if the problem already occurs at the VBSCF level : change guess