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

A complex chemical system is often examined based on their fragments, so fragment-based analysis is the key to chemical understanding. We report the natural fragment bond orbital (NFBO) method for inter-fragment bonding interaction analysis, as an extension to the well-known natural bond orbital method. NFBOs together with their corresponding natural fragment hybrid orbitals (NFHOs) allow us to derive local bonding and anti-bonding orbitals among fragments from the delocalized canonical molecular orbitals. In this paper, we provide the algorithm for finding NFBOs and showcase its application to several chemically interesting systems featuring significant inter-fragment bonding interactions. Through these examples, the NFBO method is shown to be a powerful tool with which to analyze molecules possessing strong inter-fragment bonding interactions.