General guidelines for BOVB calculations

General procedure for BOVB calculations

Ctrl options

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.

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

1. Perform a ("L") VBSCF calculation ;
2. then perform the L-BOVB calculation, always starting from converged VBSCF orbitals ;
3. then perform the D-BOVB calculation : 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, with all and only PI pairs being defined as active), then you can use the following localization space :

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

Procedure for the calculation

1. Perform a ("D") VBSCF calculation ;
2. then perform the D-BOVB calculation : starting from converged VBSCF orbitals

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%) ;
• large negative weights (<-0.05) Coulson-Chrigwin weights are also a sign of convergence on an unphysical solution ;
• the overlap matrix between 2 given structures should not exceed ~0.7
• the total energy : when an instability occur, it may become too low, leading to, for instance, dissociation energies which might be (sometimes significantly) larger than exact ones ;
• the orbital overlap (in the ".xdat" file) between active orbitals : corresponding active orbitals in different structures 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

What can I do if I encounter a BOVB instability ?

Check the following points :

• Did I eliminate structures with have minor (<1%) weight at VBSCF level from my BOVB calculation ?
• Did I use a basis set with diffuse functions (this should not be used for BOVB calculations, except when you have a bare anion), or a basis set larger than triple-zeta (too large basis set in general could cause trouble) ?
• Did I precisely follow the procedures described above to get my L/D-BOVB wave function ?

How to cure the problem - if I haven't done any particular "mistake" :

• try to modify slightly the definition of the wave-function (for instance : having active orbital strictly localized on 1 atom instead of fragment orbitals, or the opposite) ;
• try to start from the VBSCF level with another orbital guess ;
• try to work in a different basis set.