The Quality of Embedding Charges Is Critical for Convergence of Many-Body Expansions When BSSE Is Absent

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