ON THE ACCURACY OF THE ALGEBRAIC-APPROXIMATION IN MOLECULAR ELECTRONIC-STRUCTURE CALCULATIONS .5. ELECTRON CORRELATION IN THE GROUND-STATE OF THE NITROGEN MOLECULE

Many-body perturbation theory, formulated within the algebraic approxi mation, is used to determine the correlation energy of the ground stat e of the nitrogen molecule. Systematically constructed even-tempered b asis sets of Gaussian-type functions, which have been shown to achieve an accuracy approaching similar to 1 mu Hartree in matrix Hartree-Foc k calculations, have been developed to afford basis sets suitable for electron correlation studies using the second-order many-body perturba tion expansion. Over 98% of an empirical estimate of the ground-state correlation energy of the N-2 molecule at its equilibrium geometry is recovered by using a basis set constructed from even-tempered sets cen tred on the atoms and on the bond midpoint and containing functions of s, p, d, f, g and h symmetry. The importance of functions with i symm etry is also assessed. It is estimated that the calculated correlation energy corresponds to 99.1% of the exact second-order energy. The cor relation energy obtained in the present study is compared with second- order energies obtained by using previously reported basis sets, which are shown to recover, at best, about 10% less of the empirical correl ation energy estimate.