Formulae for a SO(3)-invariant expansion of the pair-molecular interaction-energy are derived. Two different cases, of exponential potential and of inverse-power potential, are considered for molecules of arbitrary shape. Methods for the determination of the generalized multipole components for molecules are developed on the base of transformation formulae for rotation and translation of a coordinate system. The importance of accounting the difference between the general expansion and that of a conventional multipole type is illustrated on the base of simulation data for mesogen substance PCH-5.