Presently I am working on new conceptual and computational model for describing the electronic structure of molecules and solids: the PATMOS model. A key element in this approach is the definition of perturbed atoms in a complex. A perturbed atom is at the lowest level of approximation described by an UHF-type wave function. However, the orbitals are not UHF orbitals. They are determined by minimizing a functional derived from generalized valence bond type corrections. The energy is calculated by a bi-variational energy incremental method. By partitioning the interaction terms, the total electronic energy can be written as a sum of additive atomic energies. Hence, the binding energy per atom in a solid can be calculated by considering only the additive energies for the atoms in the unit cell.