A technique for the calculation of current-to-voltage curves and control curves of a molecular single-electron transistor with a discrete energy spectrum has been developed. The effective recursive methods for quick computation of the Gibbs canonical distribution of electrons between energy levels, as well as techniques for the fast calculation of the distribution function for a slow relaxation process have been found. Characteristics of the single-electron transistor in the cases of different types of molecule's energy spectrum, fast and slow energy relaxations have been compared. (C) 2004 Published by Elsevier B.V.