19 ORBITAL LOCALIZATION

Localized orbitals are calculated according to the Boys, Pipek-Mezey or NLMO criteria. Localization takes place within each symmetry species separately. If complete localization is desired, no symmetry should be used. All subcommands can be abbreviated by three characters.

The localization program is invoked by the LOCALI command

LOCALI[,method],[LOCMETHOD=locmethod],[REFORB=record]

The keyword method can be either BOYS, PIPEK or NATURAL. By default, the valence orbitals from the last energy calculation are localized using the Boys criterion. Only orbital subsets which leave the energy invariant are transformed. These defaults can be modified using the optional commands described in the following sections.

The option LOCMETHOD only applies to Pipek-Mezey localization. The value locmethod can take the following values:

locmethod=1:

Standard iterative localization method

locmethod=2:

Use second-order localization method. Redundant rotations will be eliminated.

locmethod=3:

First do a first iterations using the standard method, then invoke locmethod=2. This option is recommended in cases with redundant rotations, e.g., benzene.

The option REFORB has the same effect as the directive REFORB described further below.