Expansion variables for general quartic force fields of triatomic molecules

A framework for general quartic force fields of polyatomic molecules, employing expansion coordinates of the form ρ=(r − re)/r, is developed. The new approach is shown to be superior in terms of both rate and region of convergence to traditional expansions, and a procedure for obtaining the necessary coefficients is outlined. Results are presented for CO2, CS2, HCN, and N2O, and the chemical utility of the new force fields is assessed.