Accurate ab initio quartic force fields for the N 2O and CO 2 molecules

Abstract

The quartic force fields of N 2O and CO 2 have been computed ab initio using large basis sets of spdf and spdfg quality and augmented coupled-cluster (CCSD(T)) methods. The CCSD(T)/spdf frequencies for N 2O, and the CCSD(T)/spdfg ones for CO 2, are in excellent agreement with experiment. g functions appear to be important for cumulenic double bond stretches and bends. Improving the basis from spdf to spdfg appears to shorten double bonds by ≈0.003 Å; the effect on single bonds is smaller and too unsystematic to be quantified. SCF level anharmonicities for N 2O are qualitatively incorrect: this is not the case for CO 2 where reasonable agreement with experiment is reached even at this level. Of the various published experimental force fields for N 2O and CO 2, the ones obtained by algebraic contact transformation appear to be the most reliable.