Alternative perturbation method for the molecular vibration–rotation problem

This article introduces an alternative perturbation scheme to find approximate solutions of the spectral problem for the rotation–vibration molecular Hamiltonian. The method is implemented for the Watson Hamiltonian and applied to methane. The complete J = 0 spectrum of this penta‐atomic molecule of atmospheric interest is calculated up to 9200 cm −1 in a purely ab initio fashion. Then, the rotational spectra of the vibrational ground state is obtained up to J = 18. The largest relative error is 2.10 −5 (for the highest J = 18 level) after scaling of a single parameter. Without scaling the accuracy on the rotational levels is limited by that of the ab initio equilibrium bond distance. The convergence of our results is analyzed with respect to the different parameters involved in our approach. The important concept of vibrational mean‐field configuration interaction is introduced.