On the numerical determination of Dunham's coefficients: An application to X1Σ+HCl isotopomers

Rovibrational transition energies of the centrifugally distorted oscillator for the manifold of the HCl isotopomers are computed by means of an efficient variational–numerical method. The starting points are the potential energy surface determined using highly correlated ab initio calculations (RCCSD(T), MRCI+Q) and the aug-cc-pV6Z and the aug-cc-pCV5Z basis sets. From these data, it is shown how to calculate the mechanical and spectroscopic parameters corresponding to the ground electronic state of the diatomic molecule as well as the coefficients for the Dunham expansion. The relevance of the electronic correlation for an accurate description of the system and the effect of the core correlation are also discussed.