Refusal is discussed of long wave approximation for centres-of-mass of molecules in derivation of absorption coefficient expression. In this case, additional operators appear in the absorption coefficient expression, which are related with wave vector of the field and coordinates of centres-of-mass of molecules. Their presence results in gaining of relations between absorption coefficients and frequency detunings, which can be named sums's rules. Expressions for them are derived through the commutators of additional operators with Hamiltonian of the absorbing molecule. Hypothesis of the "drift" is formulated, explaining the existence of long trajectories exceeding the bounds of an elementary volume.
long-wave approach; absorption coefficient; rule of sums