A new computational method for electronic-structure studies of molecules and solids is presented. The key element in the new model -denoted the perturbed atoms in molecules and solids model -is the concept of a perturbed atom in a complex. The basic approximation of the new model is unrestricted Hartree Fock (UHF). The UHF orbitals are localized by the Edmiston-Ruedenberg procedure. The perturbed atoms are defined by distributing the orbitals among the nuclei in such a way that the sum of the

more »

... tra-atomic UHF energies has a minimum. Energy corrections with respect to the UHF energy, are calculated within the energy incremental scheme. The most important threeand four-electron corrections are selected by introducing a modified geminal approach. Test calculations are performed on N 2 , Li 2 , and parallel arrays of hydrogen atoms. The character of the perturbed atoms is illustrated by calculations on H 2 , CH 4 , and C 6 H 6 .