Professor Valerio Tognetti Department of Physics University of Florence Largo E.Fermi, 2 50125 Florence Italy Tel. 39-55-230751 (operator, Phys. Dept.) 39-55-2307692 (office) Fax 39-55-229330 Italy visited St.Peterburg from 19 to 23 May 1996 and gave the talk at the seminar in EIMI on May, 21, 1996: "Effective potential and effective hamiltonian for statistical mechanics of condensed matter systems"