HOW LARGE IS THE EFFECT OF 1S CORRELATION ON THE D-E, OMEGA(E), AND R(E) OF N-2

The effect of N 1s correlation on the spectroscopic constants of N-2 is studied using the coupled-cluster singles and doubles approach with a perturbational estimate of the connected triples [CCSD(T)] and internally contracted multireference configuration interaction (ICMRCI) techniques. At the ICMRCI level, we obtain a Is effect of +1.35 kcal/mol on the dissociation energy. However, the effect is found to be smaller when size-extensive methods are used. The Is effects computed at the CCSD(T) and internally;contracted averaged coupled-pair:functional (ICACPF) levels are in excellent agreement. Our best estimate for the effect of Is correlation is +0.8 kcal/mol on D-e, +9 cm(-1) on omega(e), and -0.002 Angstrom on the bond length. Including our estimate for the effect of Is correlation, we obtain a D-e of 227.1 using a correlation-consistent polarized-valence sextuple zeta basis set at the ICMRCI+Q level (where the +Q indicates that the Davidson correction has been included). Basis set incompleteness, which is estimated to be 0.7+/-0.2 kcal/mol, is still the major source of error. The CCSD(T) ten-electron results are found to be in excellent agreement with those obtained at the ICACPF or ICMRCI+Q levels of theory.