A set of equations for deriving orthogonal many-electron group orbitals is deduced. This approach can be utilized to define an exact measure for the localizability of the electron groups in a system. It is shown that anN-electron model operator Open image in new window can be introduced the eigenfunctions of which are antisymmetrized products of the many-electron group orbitals. Taking Open image in new window as perturbation the first order correction vanishes and the second order correction is just the dispersion interaction between the separated groups. Taking an external electric or magnetic field as perturbation the corrections up to second order are additive by groups, i.e., they are the sum of the contributions of the separated electron groups.