Accurate abinitio quartic force-fields for the n2o and co2 molecules

Abstract

The quartic force fields of N2O and CO2 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 N2O, and the CCSD(T)/spdfg ones for CO2, 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 almost-equal-to 0.003 angstrom; the effect on single bonds is smaller and too unsystematic to be quantified. SCF level anharmonicities for N2O are qualitatively incorrect: this is not the case for CO2 where reasonable agreement with experiment is reached even at this level. Of the various published experimental force fields for N2O and CO2, the ones obtained by algebraic contact transformation appear to be the most reliable.