A novel approach to relax extremely localized molecular orbitals: The extremely localized molecular orbital-valence bond method

Orbitals rigorously defined on molecular fragments, like the extremely localized molecular orbitals, represent a possible way to build up the electronic structure of large molecules, using molecular fragments assembled on small molecules. Of course a rigorous localization is strictly connected to a reduction in the number of variational parameters, which reflects itself in an increased value of the associated energy. In order to get a more accurate description of the target molecule, we have developed a method which allows a relaxation of the electronic structure based on transferred localized orbitals. The relaxation is realized by means of a valence bond technique, which in turn uses the localized nature of the orbitals to reduce the number of excitations. Applications to the transferability of extremely localized molecular orbitals are presented.