Rotation–vibration energy levels by a Hamiltonian reexpansion technique

It is shown that by reexpanding the interaction potential for a molecule as a product of functions of the internal coordinates, the Hamiltonian matrix may be rapidly and efficiently computed and the rotation–vibration eigenvalues of the molecule may be easily computed for a number of rotational states. The Kratzer oscillator functions are found to be a rapidly converging set for this purpose. The method was tested on the HCN molecule using a potential for which accurate earlier calculations are available for comparison.