Natural localized molecular orbitals

TLDR

Natural localized molecular orbitals (NLMOs) extend the natural atomic and bond orbital framework by using only the one‑particle density matrix. The authors propose NLMOs as a novel, efficient technique for generating localized molecular orbitals for SCF and CI wave functions. The NLMO method derives LMOs directly from the density matrix, thereby preserving σ–π separation in planar molecules. Tests on methane and cytosine demonstrate that NLMOs closely match Boys and Edmiston–Ruedenberg LMOs, require only a modest fraction of SCF time, and reveal delocalization tails, thereby strengthening the link between chemical intuition and molecular wave functions.

Abstract

The method of natural localized molecular orbitals (NLMOs) is presented as a novel and efficient technique for obtaining LMOs for SCF and CI wave functions. It is an extension of the previously developed natural atomic orbital (NAO) and natural bond orbital (NBO) methods, and uses only the information contained in the one-particle density matrix. Results are presented for methane and cytosine to indicate that NLMOs closely resemble LMOs obtained by the Boys and Edmiston–Ruedenberg methods, with the exception that the NLMO procedure automatically preserves the MO σ–π separation in planar molecules. The computation time is modest, generally only a small fraction of the SCF computation time. In addition, the derivation of NLMOs from NBOs gives direct insight into the nature of the LMO ‘‘delocalization tails,’’ thus enhancing the role of LMOs as a bridge between chemical intuition and molecular wave functions.