Accurate determination of an equilibrium structure in the presence of a small coordinate: The case of dimethylsulfide

The ab initio structure of dimethylsulfide is calculated at the CCSD(T) level of theory using a basis set of quadruple-ζ quality which takes into account the core correlation. The quadratic, cubic and semi-diagonal quartic force field of dimethylsulfide is calculated at the MP2 level of theory employing a basis set of triple-ζ quality. A semi-experimental equilibrium structure is derived from experimental ground-state rotational constants and rovibrational interaction parameters calculated from the ab initio force field. In spite of a very large number of isotopologues, the position of the in-plane hydrogens is poorly defined. This is explained by the small b-coordinate of these atoms. The problem is solved using the merged least-squares method where the input data are the semi-experimental and the ab initio structures.