This keyword localizes user-defined $\pi$-groups in extended $\pi$-systems.
Options: None
Each line in the keyword body defines a $\pi$-group. During the SCF the $\pi$ interaction between these groups is suppressed. Thus, the energy difference with respect to the unconstrained calculation represents the resonance energy. The definition of the $\pi$-group consists of one or more atomic labels. These labels must be defined in the keyword body of GEOMETRY (see 4.1.1). For example, assume the following geometry definition for benzene:

 C1       0.000000      0.000000      0.000000
 C2       0.000000      0.000000      1.394129
 C3       1.207359      0.000000     -0.697056
 C4       1.207341      0.000000      2.091222
 C5       2.414698      0.000000      0.000038
 C6       2.414694      0.000000      1.394163
 H       -0.952713      0.000000      1.944171
 H       -0.952718      0.000000     -0.550037
 H        3.367421      0.000000     -0.549983
 H        3.367408      0.000000      1.944198
 H        1.207334      0.000000      3.191314
 H        1.207389      0.000000     -1.797150

Then the following LOCALIZATION input defines three localized ethylene $\pi$-groups:

 C1 C2
 C4 C6
 C3 C5

Thus, the interactions between these $\pi$-groups are suppressed. The $\sigma$-orbitals are not affected. The LOCALIZATION keyword is applicable only to planar systems. However, the system orientation is arbitrary. The LOCALIZATION and SYMMETRY keywords are incompatible and, therefore, cannot be used together in the input.