The formulae at Hartree-Fock level are derived for the energy of Coulomb interaction between molecules containing any number of closed and open shells. These formulae are expressed in terms of linear combination coefficients of molecular orbitals and multicenter integrals of Slater type orbitals and interaction potentials produced by the charges of the molecules. As an example of application, by the use of unsymmetrical one-range addition theorems of Slater type orbitals the Hartree-Fock-Roothaan calculations have been performed for the energy of interaction produced by the ground states of two molecules H2O. (C) 2010 Elsevier B.V. All rights reserved.