A general coupled cluster study of the N2 molecule

The equilibrium distance, harmonic frequency and potential curve of the nitrogen molecule are investigated using the cc-pVDZ basis and various single- and multi-reference coupled cluster (CC) methods. Including single and double excitations from all determinants of the smallest active space that ensures correct dissociation, the CC method gives deviations from full configuration interaction (FCI) of 0.0001 (A) over circle, for the equilibrium distance, 1 cm(-1) for the frequency, and a non-parallelity error (NPE) of 0.0006 E-h for the potential curve. Restricting the single and double excitations from the active space to those that are at most quadruple excited compared to the Hartree-Fock determinant, produces results that are very close to those obtained including all excitations up to quadruple excitations. (C) 2001 Elsevier Science BN. All rights reserved.