Geometry and harmonic frequency of N2 with coupled cluster methods that include connected quadruple excitations

Coupled cluster methods with full inclusion of singles, doubles, triples and noniterative connected quadruples (CCSDT(Qf)) are used to calculate the geometry and harmonic frequency of the N2 molecule for a series of correlation-consistent basis sets up to the polarized valence 5-zeta. The connected quadruple excitations lower the harmonic frequency by 19 cm−1 and raise the bond length by 0.0012 Å. For the approximate inclusion of the T3 cluster the connected quadruple correction is smaller, but consistent for the basis sets studied. The frequency value for the largest basis set with all electrons correlated (quadruple-zeta quality) is 3 cm−1 below the experimental value. The estimated basis set limit for the CCSDT(Qf) method is 2361 cm−1 compared to the experimental value of 2358.6 cm−1.