How large is the effect of 1s correlation on the D e, ωe, and r e of N2?

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Abstract
The effect of N 1s correlation on the spectroscopic constants of N2 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 1s 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 1s 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 1s correlation is +0.8 kcal/mol on De, +9 cm−1 on ωe, and −0.002 Å on the bond length. Including our estimate for the effect of 1s correlation, we obtain a De 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.