A previously proposed microscopical method to calculate deformation energies of heavy nuclei, using Skyrme type effective interactions, is reinvestigated. It is shown that if the effective mass is included in the phenomenolozical one body Hamiltonian, whose eigenfunctions are used to calculate the expectation value of the total Skyrme Hamiltonian, one can obtain deformation energies very close to the ones obtained in constrained Hartree-Fock calculations.Since the re-discovery [1] of the effective nucleonnucleon force of Skyrme [2], constrained HartreeFock (CHF) calculations for heavy nuclei have become technically possible [3,4]. However, such calculations require large amounts of computer time, so that it would be far too time consuming to include nonaxial and left-right asymmetric deformations in a systematical investigation of fission barriers using the CHF method.A purely microscopical, but less time consuming approximation to the CHF method was proposed some years ago by Ko et al. [5]. The main idea of this approach is to utilize the single particle wave functions of the deformed Woods-Saxon (WS) potential, determined in Strutinsky type fission barrier calculations [6,7], to calculate the expectation value of the total Skyrme Hamiltonian:Here OSky is the Skyrme interaction and (bws(13i) a Slater determinant built of the WS single particle wave functions ~pv(r, 3i) which depend on one or more deformation parameters 3# The quadrupole moment Q2 and higher moments can easily be calculated from the ~Pv, too. Thus, eq. (1) gives directly the total energy of a nucleus as a function of deformation E(3i) or E(Q 2 ; ...), and the parameters 3i play the role of the constraint in the CttF method ~. * Work supported in parts by USERDA contract E(11-1)-3001. 1 Present address: Institut Laue Langevin, F-38042 Grenoble, France. 2 Laboratoire associ~ au C.N.R.S. $ For simplicity, we omit the indices for protons and neutrons.The results found with this expectation value method (EVM) were only partially successful [5 ]. Whereas the shell structure in the deformation energies was reasonably well reproduced, their average part increased too much at large deformations. In 240pu, e.g., the second fission barrier was found more than twice as large as the one obtained in a CHF calculation with the same force.We have reinvestigated the EVM with essentially three alterations:(1) The effective masses m*(r) which for Skyrme forces differ from the free nucleon masses in the interior of the nucleus, have been included in the one body Hamiltonian.We thus solve the equations The variable fir, RO) is defined such that firs, RO) = 0 239