MOLPRO

39 THE VB PROGRAM CASVB 285
39.9.5 Defining several optimizations
More than one optimization may be performed in the same CASVB deck, by the use of OPTIM
keywords:
OPTIM[;. . . ;FINOPTIM];
The subcommands may be any optimization declarations defined in this section, as well as
any symmetry or constraints specifications described in section 39.10. Commands given as
arguments to OPTIM will be particular to this optimization step, whereas commands specified
outside will act as default definitions for all subsequent OPTIM keywords.
If only one optimization step is required, the OPTIM keyword need not be specified.
When only a machine-generated guess is available, CASVB will attempt to define a sequence of
optimization steps chosen such as to maximize the likelihood of successful convergence and to
minimize CPU usage. To override this behaviour, simply specify one or more OPTIM cards.
39.9.6 Multi-step optimization
A loop over two or more optimization steps may be specified using:
ALTERN,Niter;. . . ;FINALTER
With this specification the program will repeat the enclosed optimization steps until either all
optimizations have converged, or the maximum iteration count, Niter, has been reached.
39.10 Point group symmetry and constraints
The problems associated with symmetry-adapting valence bond wavefunctions are considered,
for example, in: T. Thorsteinsson, D. L. Cooper, J. Gerratt and M. Raimondi, Theor. Chim. Acta
95, 131 (1997).
39.10.1 Symmetry operations
SYMELM,label,sign;
Initiates the definition of a symmetry operation referred to by label (any three characters). sign
can be + or −; it specifies whether the total wavefunction is symmetric or antisymmetric under
this operation, respectively. A value for sign is not always necessary but, if provided, constraints
will be put on the structure coefficients to ensure that the wavefunction has the correct overall
symmetry (note that the configuration list must be closed under the orbital permutation induced
by label for this to be possible).
The operator is defined in terms of its action on the active MOs as specified by one or more of
the keywords IRREPS, COEFFS, or TRANS (any other keyword will terminate the definition
of this symmetry operator). If no further keyword is supplied, the identity is assumed for label.
The alternative format SYMELM,label,sign;key-1,. . . ;key-2,. . . ;. . . may also be used.
39.10.2 The IRREPS keyword
IRREPS,i 1 , i 2 ,. . . ;