Accurate ab initio potential energy curve of O2. I. Nonrelativistic full configuration interaction valence correlation by the correlation energy extrapolation by intrinsic scaling method.

The recently introduced method of correlation energy extrapolation by intrinsic scaling is used to calculate the nonrelativistic electron correlations in the valence shell of the O(2) molecule at 24 internuclear distances along the ground state (3)Sigma(g)(-) potential energy curve from 0.9 to 6 A, the equilibrium distance being 1.207 52 A. Using Dunning's correlation-consistent triple- and quadruple-zeta basis sets, the full configuration interaction energies are determined, with an accuracy of about 0.3 mhartree, by successively generating up to sextuple excitations with respect to multiconfigurational reference functions that strongly change along the reaction path. The energies of the reference functions and those of the correlation energies with respect to these reference functions are then extrapolated to their complete basis set limits.