Approximations for angular momentum-conserved polyatomic vibrational-rotational sum and density of states under a central potential

An approximation is suggested for the calculation of the angular momentum-conserved rotational sum of states for polyatomic molecules under a central potential, based on an interpolation between high-J and low-J forms (J = total angular momentum). A similar procedure is suggested for an approximation to the angular momentum-conserved polyatomic vibrational-rotational sum of states, which obviates a computationally intensive convolution integral. The procedure is checked against rigorous results with satisfactory results. It can be applied to various fragment symmetries in both the phase space limit and at arbitrary interfragment distance, which is of some interest in variational routines.