A method to include certain infinite order contributions in a multiconfigurational self-consistent field (MCSCF) calculation

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Abstract
The independent variables of unitary transformation are transformed into two new groups. Using these new variables, we derive an exact MC energy expression. One group of variables is related to the modes of orbital or configurational mixing. These are approximately determined by using first or second order information of an energy hypersurface. The other group is related to the magnitude of orbital or configurational mixing. These are exactly determined, i.e., through infinite order. With this procedure, we can investigate the structure of the energy hypersurface and more safely approach the desired stationary point when far from convergence. We show the results of MCSCF calculations with these infinite order procedures on the ground states of C2 and CO.