The density of states (DS) of a disordered system with localised electronic states is studied in the vicinity of the Fermi level with the Monte-Carlo computer simulation for the two- and three-dimensional simple model. The minimisation of the total energy with respect to all one-electron transitions is shown to be a good approximation both for the total energy and for the DS. The electron-electron interaction drastically changes the DS in the vicinity of the Fermi level. The DS is shown to have a 'soft' Coulomb gap, and the self-consistent equation fits the results of simulation well. The finite size effect is also studied.