The ORTHCON keyword initiates the input of orthogonality constraints between pairs of valence bond orbitals. The sub-keywords key-i can be one of ORTH, PAIRS, GROUP, STRONG or FULL as described below. Orthogonality constraints should be used with discretion. Note that orthogonality constraints for an orbital generated from another by symmetry operations (using the ORBREL keyword) cannot in general be satisfied.
ORTH,i, i, ...;
Specifies a list of orbitals to be orthogonalized. All overlaps between pairs of orbitals in the list are set to zero.
PAIRS,i, i, ...;
Specifies a simple list of orthogonalization pairs. Orbital is made orthogonal to , to , etc.
GROUP,label,i, i, ...;
Defines an orbital group to be used with the ORTH or PAIRS keyword. The group is referred to by label which can be any three characters beginning with a letter a-z. Labels defining different groups can be used together or in combination with orbital numbers in ORTH or PAIRS. i, i, ... specifies the list of orbitals in the group. Thus the combination GROUP,AZZ,1,2; GROUP,BZZ,3,4; ORTH,AZZ,BZZ; will orthogonalize the pairs of orbitals 1-3, 1-4, 2-3 and 2-4.
This keyword is short-hand for strong orthogonality. The only allowed non-zero overlaps are between pairs of orbitals (, ).
This keyword is short-hand for full orthogonality. This is mainly
likely to be useful for testing purposes.