MOLPRO

39 THE VB PROGRAM CASVB 287
39.10.7 Freezing orbitals in the optimization
FIXORB,i 1 , i 2 ,. . . ;
This command freezes the orbitals specified in the list i 1 , i 2 ,. . . to that of the starting guess.
Alternatively the special keywords ALL or NONE may be used. These orbitals are eliminated
from the optimization procedure, but will still be normalized and symmetry-adapted according
to any ORBREL keywords given.
39.10.8 Freezing structure coefficients in the optimization
FIXSTRUC,i 1 , i 2 ,. . . ;
Freezes the coefficients for structures i 1 , i 2 ,. . . . Alternatively the special keywords ALL or
NONE may be used. The structures are eliminated from the optimization procedure, but may
still be affected by normalization or any symmetry keywords present.
39.10.9 Deleting structures from the optimization
DELSTRUC,i 1 , i 2 ,. . . ,[ALL],[NONE];
Deletes the specified structures from the wavefunction. The special keywords ALL or NONE
may be used. A structure coefficient may already be zero by symmetry (as defined by SYMELM
and ORBREL), in which case deleting it has no effect.
39.10.10 Orthogonality constraints
ORTHCON;key-1,. . . ;key-2,. . . ;. . .
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 1 , i 2 , . . . ;
Specifies a list of orbitals to be orthogonalized. All overlaps between pairs of orbitals in the list
are set to zero.
PAIRS,i 1 , i 2 , . . . ;
Specifies a simple list of orthogonalization pairs. Orbital i 1 is made orthogonal to i 2 , i 3 to i 4 ,
etc.
GROUP,label,i 1 , i 2 , . . . ;
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 1 , i 2 , . . . 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.
STRONG;