| Abstract: | Significant improvements in calculational efficiency and capability with the self-consistent electron pairs (SCEP ) method has resulted from several new computational developments. A new procedure for constructing the important internal Coulomb and exchange operators has substantially reduced the preiteration time. A general scheme for utilizing molecular symmetry has been used to advantage in reducing the number of pair functions and external operators that must be found explicitly at each iteration. A projection operator tool has been implemented and found to be quite effective at minimizing the number of iterations required at some point on a potential energy surface when an SCEP wavefunction exists for some nearby point. These and other improvements in the program construction have yielded sizable reductions in time for some representative test cases, including water and a potential energy curve for formaldehyde. The new SCEP program also performs low-order perturbation theory treatments and coupled electron pair approximation (CEPA ) calculations using the same operator approach. The usefulness of the approach is demonstrated by very large scale calculations on the stability of the two interstellar glycine conformers. These calculations involve the variational treatment of 82,205 symmetry-adapted singly and doubly substituted configurations involving 225 internal electron pairs. |
