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(May 16, 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