Challenging High-Level ab initio Rovibrational Spectroscopy: The Nitrous Oxide Molecule

Abstract

The equilibrium structure and rovibrational energies of nitrous oxide (N2O) in its electronic ground state (X-1 Sigma(+)) are derived from a high-level ab initio potential energy function (PEF). This PEF is based on a composite approach with the basic contribution given by explicitly correlated coupled-cluster (CC) calculations. Smaller contributions include corrections due to inner-shell correlation, scalar-relativistic effects and higher-order correlation up to iterative pentuple excitations (CCSDTQP in CC nomenclature). The high importance of higher-order correlation in order to reach the desired accuracy led to the use of an extrapolation scheme to approximately account for the effect of hextuple and some pentuple excitations. A reasoning for the soundness of the method is given in this work. The results of the rovibrational calculations are compared to those of two multi-reference (MR) based composite PEFs, where the basic contribution is given by MR configuration interaction and MR average coupled-pair functional calculations. A highly accurate electric dipole moment function is also computed by the three composite methods in excellent agreement with the experimental values available. Subtle irregularities in the intensity pattern are reproduced in great detail and several kinds of resonances are analyzed without the need to empirically adjust our best ab initio PEF. The equilibrium bond lengths were determined by a mixed experimental/theoretical approach yielding R-e (NN) = 1.12695 (10) angstrom and R-e(NO) = 1.18539(5) angstrom.