MULTIREFERENCE PERTURBATION CI - II - SELECTION OF THE ZERO-ORDER SPACE

In this paper, the second of a series devoted to multi-reference pertu rbation CI, we tackle the problem of the appropriate selection of the zero-order space in CIPSI calculations. We propose a new selection pro cedure, explicitly devised in order to obtain a balanced description f or different electronic states and nuclear geometries. To this aim, we define numerically the quality of the zero-order space by means of a suitable parameter sigma, which is the square norm of the perturbative correction of the wavefunction. The zero-order space is expanded step wise so as to obtain the same target sigma for all states and geometri es. This strategy is applied to the calculation of dissociation, activ ation and transition energies. It yields a much better convergence of perturbative and zero-order results, when compared with the selection procedure previously used.