SIMPLE RECIPE FOR IMPLEMENTING COMPUTATION OF FIRST-ORDER RELATIVISTIC CORRECTIONS TO ELECTRON CORRELATION ENERGIES IN FRAMEWORK OF DIRECT PERTURBATION-THEORY

The computation of the relativistic correction to the first order in 1 /c(2), where c is the velocity of light, is implemented at the levels of coupled cluster and many-body perturbation theory. The relativistic correction is obtained by applying direct perturbation theory through the first order, and it is shown that its implementation is straightf orward if analytical energy gradients of the methods under considerati on are available. Preliminary results were obtained by a numerical pro cedure and are reported for some closed-shell atoms (He, Be, Ne, and A r) and molecules (CuH and SiH4). (C) 1997 by John Wiley & Sons. Inc.