Accurate ab initio prediction of the equilibrium geometry of HCO⁺ and of rovibration energy levels of DCO⁺

Abstract

The ground state potential energy surface for HCO⁺ has been investigated theoretically. A large fraction of the electron correlation is included by multi reference internally contracted configuration interaction from CASSCF reference wavefunctions using large orbital expansions. The origin of the potential is then shifted and the force constants scaled to reproduce all spectroscopic data available for five isotopically substituted species. The estimated equilibrium geometry and force constants agree closely with the results of previous computations. Variational calculations of vibrational and rotational frequencies for transitions up to J = 4 ⇃ 3 have been performed, with accuracy which is typically ± 2 cm^-1 for vibrations and ± 20 MHz for most rotations. From computed and experimentally available ground state rotational and l-doubling constants estimates have been made of the equilibrium geometry (r _e(C-H) = 0·0919 ± 0·0009 Å and R _e (C-O) = 1·1055 ± 0·0003 Å) and of the most important anharmonic interactions.