MOLPRO

19 THE MCSCF PROGRAM MULTI 129
19 THE MCSCF PROGRAM MULTI
MULTI is a general MCSCF/CASSCF program written by
P. J. Knowles and H.-J. Werner (1984).
Bibliography:
H.-J. Werner and P. J. Knowles, J. Chem. Phys. 82, 5053 (1985).
P. J. Knowles and H.-J. Werner, Chem. Phys. Lett. 115, 259 (1985).
All publications resulting from use of this program must acknowledge the above. See also:
H.-J. Werner and W. Meyer, J. Chem. Phys. 73, 2342 (1980).
H.-J. Werner and W. Meyer, J. Chem. Phys. 74, 5794 (1981).
H.-J. Werner, Adv. Chem. Phys. LXIX, 1 (1987).
This program allows one to perform CASSCF as well as general MCSCF calculations. For
CASSCF calculations, one can optionally use Slater determinants or CSFs as a N-electron basis.
In most cases, the use of Slater determinants is more efficient. General MCSCF calculations
must use CSFs as a basis.
A quite sophisticated optimization method is used. The algorithm is second-order in the orbital
and CI coefficient changes and is therefore quadratically convergent. Since important higher
order terms in the independent orbital parameters are included, almost cubic convergence is
often observed. For simple cases, convergence is usually achieved in 2-3 iterations. However,
convergence problems can still occur in certain applications, and usually indicate that the active
space is not adequately chosen. For instance, if two weakly occupied orbitals are of similar
importance to the energy, but only one of them is included in the active set, the program might
alternate between them. In such cases either reduction or enlargement of the active orbital space
can solve the problem. In other cases difficulties can occur if two electronic states in the same
symmetry are almost or exactly degenerate, since then the program can switch from one state
to the other. This might happen near avoided crossings or near an asymptote. Problems of this
sort can be avoided by optimizing the energy average of the particular states. It is also possible
to force convergence to specific states by choosing a subset of configurations as primary space
(PSPACE). The hamiltonian is constructed and diagonalized explicitly in this space; the coefficients
of the remaining configurations are optimized iteratively using the P-space wavefunction
as zeroth order approximation. For linear molecules, another possibility is to use the LQUANT
option, which makes it possible to force convergence to states with definite Λ quantum number,
i.e., Σ, Π, ∆, etc. states.
19.1 Structure of the input
All sub-commands known to MULTI may be abbreviated by four letters. The input commands
fall into several logical groups; within each group commands may appear in any order, but the
groups must come in correct order.
a) The program is invoked by the command MULTI or MCSCF
b) cards defining partitioning of orbitals spaces – OCC,FROZEN,CLOSED
c) general options (most commands not otherwise specified here)
d) a WF card defining a state symmetry
e) options pertaining to that state symmetry – WEIGHT,STATE,LQUANT