Matrix elements of doubly degenerate vibrations and the Herzberg-Teller series

► Simplified formulae for matrix elements/transition moments between l-components of doubly degenerate vibrations in upper and lower electronic states. ► Cancelation effects for large change of frequency, especially for second-order Herzberg-Teller terms. ► Clarification of some assignments/absences in the benzene A∼1B2u-X∼1A1g system.