An analytical formula is proposed for approximation of resonance functions in the broadening theory and for shift of molecular spectral lines. This formula accounts for the trajectory curvatures of colliding particles. The equation obtained is a part of a power series in terms of hyperbolic tangents th(z). The coefficients of this series are determined for the case of trajectories defined by the Lennard-Jones (6-12) intermolecular interaction potential. This paper presents the coefficient values for the case of electrostatic interactions. Real parts of the resonance functions needed for calculation of broadening coefficients are considered.