MOLPRO

19 THE MCSCF PROGRAM MULTI 146
19.9.4 MCSCF hessians
The MCSCF/CASSCF hessian can be computed analytically (only without symmetry) using
CPMCSCF,HESS,[ACCU=value]
where the ACCU option specifies the convergence threshold in the CPMCSCF calculation (default
1.d-4). The hessian is stored on record 5300.2 and can be used in a subsequent frequency
calculation.
Example:
{multi;cpmcscf,hess,accu=1.d-5}
frequencies
Note that the NOEXTRA option is used when computing a hessian.
19.10 Optimizing valence bond wavefunctions
VB={...}
Using this keyword, the optimization of the CI coefficients is carried out by CASVB. The VB
keyword can be followed by any of the directives described in section 39. Energy-based optimization
of the VB parameters is the default, and the output level for the main CASVB iterations
is reduced to −1.
19.11 Hints and strategies
MCSCF is not a “black box” procedure like SCF! For simple cases, for example a simple
CASSCF with no CLOSED orbitals, this program will converge in two or three iterations. For
more complicated cases, you may have more trouble. In that case, consider the following:
• Always start from neighbouring geometry orbitals when available (this is the default).
• The convergence algorithm is more stable when there are no CLOSED orbitals, i.e., orbitals
doubly occupied in all configurations, but fully optimized. Thus a reasonable approach
is to make an initial calculation with CLOSED replaced by FROZEN (all doubly
occ. frozen).
• If still no success, you can switch off the coupling between CI coefficients and orbital
rotations for a few iterations, e.g.:
{ ITERATIONS;DO,UNCOUPLE,1,TO,2; }
and/or disable the simultaneous optimization of internal orbitals & CI, e.g.:
{ ITERATIONS;DONT,INTERNAL,1,TO,2; }
You can often get a clue about where the program starts to diverge if you include:
IPRINT,MICRO;
in the data. Also consider the general remarks at the beginning of this chapter. For the details of
the algorithms used, see J. Chem. Phys 82, 5053 (1985); Chem. Phys. Letters 115, 259 (1985);
Advan. Chem. Phys. 59, 1 (1987);