20.11 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 CORE (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;UNCOUPLE,1,TO,2;END;

  and/or disable the simultaneous optimization of internal orbitals & CI, e.g.:

  ITERATIONS;DONT,INTERNAL,1,TO,2;END;

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);