Convergence studies in configuration interaction calculations

Abstract A series of CI calculations for the ground state of BH 3 in which various levels of excitation from the ground state are included are compared with each other and with the full‐ CI (196 configurations) result. The comparisons cover calculations in terms of two different molecular orbital bases—the canonical SCF basis and a simple, arbitrarily chosen, symmetry orbital basis. As expected, single and, to a lesser extent, triple excitations are of little importance in the SCF case but cannot be ignored for the arbitrary basis. However, as soon as all excitations ≦4 are included, there is practically no difference in the results for the two bases, both giving energies quite close to the full‐ CI value. In fact, the energies for the two bases are in close agreement also in the (0 + 1 + 2 + 3)‐excitation calculation and in qualitative agreement in the (0 + 1 + 2) case. Two methods are tested for the selection of the important higher‐excitation configurations, and it is found that results very close to full CI can be obtained with substantially fewer functions. Particularly promising is the application of the “unlinked cluster” approach, based on the ideas developed by Sinanoğlu and others, to the prediction of the coefficients of evenly excited configurations from those of the doubly‐excited functions in a limited‐ CI calculation.