The Quality of\nEmbedding Charges Is Critical for Convergence\nof Many-Body Expansions When BSSE Is Absent

There\nis conflicting evidence in the literature concerning the\nbenefits of charge embedding on the convergence of many-body expansions\n(MBEs). Using a systematic series of water and ion–water clusters\nof varying size, this study indicates that the effects of charge embedding\ncan be masked by basis set superposition error (BSSE). When BSSE is\nremoved, this study demonstrates that charge embedding can significantly\naccelerate MBE convergence, where the electrostatically embedded two-body\nmethod, EE-MBE(2), can often yield accuracy close to the four-body\nmethod, MBE(4). Contrary to previous studies on smaller systems, this\nwork shows that the performance of EE-MBE is highly sensitive to the\ncharge model, with the best performance obtained when the natural\npopulation analysis (NPA) charge model is used and generated at the\nsame level of theory used in the subsystem and supersystem calculations.\nIt was demonstrated that the “3c” composite method,\nPBEh-3c, yields NPA atomic charges that are in excellent agreement\nwith those obtained from supersystem density functional theory calculations.\nThe linear-scaling X-Polarization method provides a more general approach\nto estimating these supersystem QM atomic charges, but its performance\ndepends on how the fragments are defined.