Direct determination of fragment localized molecular orbitals and the orthogonality constraint

Approximate solution of the Hartree-Fock-Roothaan (HFR) equation is sought satisfying the criteria that (i) each molecular orbital (MO) is localized onto a predefined fragment consisting of a subset of atomic orbitals (AOs) and (ii) the MOs are orthogonal to each other as much as possible. The fragments are allowed to share AOs, permitting one to satisfy these two criteria which could be in conflict. The accuracy of the approximation depends on the size of selected fragments. (C) 2003 Elsevier Science B.V. All rights reserved.