General guidelines for BOVB calculations

General procedure for BOVB calculations

General case

Definition of the localization space

• 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 only 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

Case n°2 : 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 that 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've had 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