MODIFIED CONFIGURATION-INTERACTION METHOD FOR ACCURATE CALCULATIONS WITH SMALL BASIS-SETS

A very efficient modified configuration interaction method is introduced for accurate atomic and molecular calculations with basis sets that are substantially smaller than those used by conventional configuration interaction (CI). The energy of the ground state of helium, for example, is obtained with a relative error of 3 x 10(-8) with a total of only 393 basis functions (no extrapolations). This is accomplished by means of (i) a radial representation that converges as N-r(-9) where N-r is the number of two-electron radial functions and (ii) N-(l over bar) generalized two-electron angular functions with which the energy converges as N-(l over bar)(-6). (The standard functions of conventional CI converge as N-r(-2.9) and N-l(-3)).