On the representation of potential energy surfaces of polyatomic molecules in normal coordinates

We present a method for reduced, piecewise fitting of ab initio potential energy data for use in rovibrational calculations of polyatomic molecules. The procedure involves fitting of the 1-, 2-, 3-, and 4-mode potential fragments by a least squares refinement of polynomial terms which are automatically generated by the geometries of the data points. The method is tested for H2CS, for which we have an analytical six-dimensional potential energy surface, which is used to generate potential data in order to mimic the interface between the variational code MULTIMODE and electronic structure packages.