A SELF-CONSISTENT, NON-DIVERGENT EVALUATION OF 2ND-ORDER CORRELATION ENERGIES

In the evaluation of the correlation by means of second-order perturba tion theory, the perturbative evaluation of the coefficient of each do ubly excited configuration is replaced by the diagonalization of a min imally dressed 2 x 2 matrix. The dressing incorporates the effect of h igh-order exclusion principle violating diagrams in a self-consistent way. An efficient computation of those coefficients goes through the s torage of intermediate summations in three-dimensional arrays. This me thod, the cost of which is only a few MP2 calculations, does not diver ge when single bonds are broken, as shown on a few examples (Li2, F2). Its generalization to complete active space CI for multiple bond brea kings (and even to selected CIs) is straightforward.