10.4 Orbital Localization

10.4.1 Virtual Orbital Localization

(June 30, 2021)

Virtual orbitals can be advantageous to be localized in many scenarios. One scenario where this is useful is generalized valence bond (GVB) methods, where each bonding orbital is paired with its antibonding orbital through Sano procedure. Currently this is done in GVBMAN when PP or CCVB is run. An improved guess has been proposed that has been shown to converge faster . The new subroutine is a stand-alone version that can generate these antibonding orbitals and exit without initiating a GVB calculation. It can do Boys, Pipek-Mezey, or Edmiston-Rudenberg localization for the occupied space depending on GVB_LOCAL = 1, 2, or 3, respectively, while 0 performs it on the canonical orbitals. The subroutine also prints out each occupied orbital’s Mulliken charge, delocalization measure, and variance, in which it automatically detects the bonding orbitals and generates an antibonding guess for each. A population analysis based on this effective minimal basis can also be done using EDA_POP_ANAL = 1. The number of bonds can be enforced by taking the highest GVB_N_PAIRS specified, with no guarantee of them being bonding, i.e. they can be core or lone pairs. This is currently implemented for Restricted and Restricted Open-Shell, while Unrestricted case is underway.

ANTIBOND
       Triggers Antibond subroutine to generate antibonding orbitals after a converged SCF
TYPE:
       INTEGER
DEFAULT:
       0
OPTIONS:
       0 Does not localize the virtual space. 1 Localizes the virtual space, one antibonding for every bond. 2,3 Fill the virtual space with antibonding orbitals-like guesses. 4 Does Frozen Natural Orbitals and leaves them on scratch for future jobs or visualization.
RECOMMENDATION:
       None

DOMODSANO
       Specifies whether to do modified Sano or the original one
TYPE:
       INTEGER
DEFAULT:
       0
OPTIONS:
       0 Does original Sano procedure (similar to GVBMAN). 1 Does an improved Sano procedure that’s more localized. 2 Does another variation of Sano.
RECOMMENDATION:
       1 is always better