Natural Fragment Bond Orbital Method for Inter-Fragment Covalent Interaction Analysis

We report the natural fragment bond orbital (NFBO) method for inter-fragment covalent interaction analysis, as an extension to the well-known natural bond orbital method. NFBOs together with their corresponding natural fragment hybrid orbitals (NFHOs) can be used to highlight the local covalent bonding and anti-bonding among fragments out of the delocalized canonical molecular orbitals, from ordinary localized two-fragment bonds to delocalized multi-fragment ones. In this paper, we provide the algorithm for finding NFBOs and showcase its usage with several chemically interesting systems featuring significant inter-fragment covalent interaction. This paper also includes discussion on the relationship and differences between the NFBO method and other methods which serve similar purposes. With examples and comparisons, the NFBO method is shown to be a powerful tool with which to analyze molecules possessing strong inter-fragment covalent interaction.