SIMPLE EMPIRICAL FORMULAS AND GOOD QUALITY ESTIMATIONS FOR ELECTRON CORRELATION ENERGIES OF MOLECULES AND MOLECULAR CLUSTERS - FIRST-ROW ATOM MOLECULES

Using the ab initio coupled cluster method, correlation energies were calculated for a number of molecules composed of first-row atoms. The results of computations can be fitted rather well with simple analytic formulas. The main result of the present investigation is that intraa tomic part of the correlation energy is proportional to sum of squares of valence electron charges on atoms composing the molecule. The prop ortionality coefficient depends on the basis set used. Independently, the approximations were introduced which allow for good estimation of intraatomic correlation energy by using Moller-Plesset second-order pe rturbation calculations only. (C) 1996 John Wiley & Sons, Inc.