MOLPRO

39 THE VB PROGRAM CASVB 286
The list i 1 , i 2 ,. . . specifies which irreducible representations (as defined in the CASSCF wavefunction)
are antisymmetric with respect to the label operation. If an irreducible representation
is not otherwise specified it is assumed to be symmetric under the symmetry operation.
39.10.3 The COEFFS keyword
COEFFS,i 1 , i 2 ,. . . ;
The list i 1 , i 2 ,. . . specifies which individual CASSCF MOs are antisymmetric with respect to
the label operation. If an MO is not otherwise specified, it is assumed to be symmetric under the
symmetry operation. This specification may be useful if, for example, the molecule possesses
symmetry higher than that exploited in the CASSCF calculation.
39.10.4 The TRANS keyword
TRANS,n dim , i 1 , . . . i ndim , c 11 , c 12 , . . . c ndim n dim
;
Specifies a general n dim × n dim transformation involving the MOs i 1 , . . . i ndim , specified by the
c coefficients. This may be useful for systems with a two- or three-dimensional irreducible
representation, or if localized orbitals define the CASSCF wavefunction. Note that the specified
transformation must always be orthogonal.
39.10.5 Symmetry relations between orbitals
In general, for a VB wavefunction to be symmetry-pure, the orbitals must form a representation
(not necessarily irreducible) of the symmetry group. Relations between orbitals under the
symmetry operations defined by SYMELM may be specified according to:
ORBREL,i 1 , i 2 , label1, label2,. . . ;
Orbital i 1 is related to orbital i 2 by the sequence of operations defined by the label specifications
(defined previously using SYMELM). The operators operate right to left. Note that i 1 and i 2 may
coincide. Only the minimum number of relations required to define all the orbitals should be
provided; an error exit will occur if redundant ORBREL specifications are found.
39.10.6 The SYMPROJ keyword
As an alternative to incorporating constraints, one may also ensure correct symmetry of the
wavefunction by use of a projection operator:
(NO)SYMPROJ[,irrep 1 ,irrep 2 ,. . . ];
The effect of this keyword is to set to zero coefficients in unwanted irreducible representations.
For this purpose the symmetry group defined for the CASSCF wavefunction is used (always a
subgroup of D 2h ). The list of irreps in the command specifies which components of the wavefunction
should be kept. If no irreducible representations are given, the current wavefunction
symmetry is assumed. In a state-averaged calculation, all irreps are retained for which a nonzero
weight has been specified in the wavefunction definition. The SYMPROJ keyword may
also be used in combination with constraints.