Nuclear shells correspond to an inhomogeneous distribution of nucleons in phase space, whereas the distribution in quasi-classical phenomenological models (the liquid in phase space, is supposed to be homogeneous. Starting from this point, Nilsson's level scheme is used to calculate the shell-model correction to the "liquid drop energy" of the nucleus as a function of the occupation number and deformation. A strong correlation between the shell correction and nucleon level density at the Fermi energy was observed. In magic and mid-shell nuclei the calculated deformation energy oscillates around the LDM value. Discussion of problems related to the deformation energy such as nuclear deformations, shell effects in nuclear masses, in deformed nuclei and in nuclear fission, etc. is presented. The role of nucleon pairing is discussed.