Second-order Brillouin-Wigner perturbation theory: size-extensivity correction

A convergence study of intensities of transitions from vibrational ground state to the lower lying states is done with respect to the rank of the dipole moment surface (DMS) in the Taylor series expansion of the DMS. The relative roles of the mechanical and electrical anharmonicity are analyzed in the calculation of the intensities of vibrational transitions from ground state. We find that at least a quadratic expansion of the dipole moment functional is necessary to predict the intensities of vibrational transitions. The mechanical anharmonicity becomes important when the resonances between the vibrational states are significant.