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.