Decomposition Analysisfor Visualization of NoncovalentInteractions Based on the Fragment Molecular Orbital Method

Many-body expansions of the electron density and Fock matrix in the fragment molecular orbital method (FMO) are used to reveal the role of polarization and charge transfer on noncovalent interactions (NCI). In addition to the physicochemical insight gained from these analyses, the use of FMO permits a rapid evaluation of electron densities to study NCI. The developed method is applied to a solvated sodium cation and a small polypeptide, validating the accuracy of the approach with respect to full calculations and revealing the role of polarization and charge transfer in NCI. In order to show the full potential of the approach, the FMO/NCI method is applied to a complex of the Trp-cage (PDB: 1L2Y) protein with a ligand, delivering fruitful insights into binding from both density and energy perspectives. NCI is shown to provide a comprehensive visual picture of interactions that might be missed without it, in particular, interactions between functional groups in a fragment.