A new method is developed for the direct calculation of the inner-sphere reorganization energy via an accurate potential energy function and the vibrational spectroscopic experimental data in this paper. The dependence of various force constants (f, g, and j) on the molecular vibrational and rotational spectroscopic constants (r(e), w(e), w(e)x(e), alpha(e), B-e, and D-e) has been established in two ways. The coefficients and exponential factor included in the corresponding potential function formula have been determined. The adequacy of the diatomic molecular potential function proposed in this paper is discussed. The results of the inner-sphere reorganization energies are calculated for 20 gaseous diatomic molecules and compared with those from George-Griffith’s formula and other calculational methods.