Multireference perturbation CI II. Selection of the zero-order space

Abstract.

In this paper, the second of a series devoted to multi-reference perturbation CI, we tackle the problem of the appropriate selection of the zero-order space in CIPSI calculations. We propose a new selection procedure, explicitly devised in order to obtain a balanced description for 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 σ, which is the square norm of the perturbative correction of the wavefunction. The zero-order space is expanded stepwise so as to obtain the same target σ for all states and geometries. This strategy is applied to the calculation of dissociation, activation and transition energies. It yields a much better convergence of perturbative and zero-order results, when compared with the selection procedure previously used.