The methods of calculating the energy levels of multielectron, multiply charged ions based on a perfected model of the Bohr atom are reviewed. In addition to the electrostatic interaction, account is taken of the electrons in different orbits in the form of interaction of electric currents. The interaction constants for different electron configurations are chosen empirically. The calculated energy levels of He-, Li-, Be-, and B-like ions agree with the experimental values with a relative accuracy of 10^–3–10^–4. An application of this method of calculation to atoms at constant volume is presented. A semiempirical equation of state of solids is derived, starting with a model of an isolated atomic cell. By using the experimental values of the bulk moduli, the effective energy of the unit atomic cell is calculated for more than 100 elements and compounds. Attention is called to the fact that different phases of solids differ not only in crystal structure, but also as a rule in the electronic states of the atoms.