Further tests of the approximate double substitution coupled-cluster method

Comparisons are made between the approximate form (ACCD ) of double substitution coupled-cluster theory and the complete form (CCD) in the evaluation of total correlation energies, molecular structures, vibrational frequencies, bond lengths, dissociation energies, permanent electrical moments, and electric field gradients. Though ACCD may recover as little as half of the higher-order correlation energy when that energy is small, it does recover a very high percentage when that energy is large. ACCD and CCD potential surfaces tend to have similar shape, and for many molecular properties, the results show ACCD to be very nearly equivalent to CCD.