Maximum locality in occupied and virtual orbital spaces using a least-change strategy.

A new strategy is introduced for obtaining localized orthonormal Hartree-Fock (HF) orbitals where the underlying principle is to minimize the size of the transformation matrix from the atomic orbital basis to the HF optimized orbital basis. The new strategy gives both localized occupied and localized virtual orbital spaces. The locality of the occupied orbital space is similar to one obtained using standard localization schemes. For the virtual space, standard localization schemes fail to give local orbitals while the new strategy gives a virtual space which has a locality similar to the one of a Lowdin orthonormalization of the atomic orbital basis. Since Lowdin orthonormalization gives the most local orthonormal basis functions in the sense that they have the largest similarity with the local atomic basis functions, the new strategy thus allows the orthonormal basis to become optimized without introducing significant delocalization.