Many-body exchange-overlap interactions in rare gases and water
2014
Generalized-gradient approximations (GGAs) of density-functional theory can suffer from substantial many-body errors in molecular systems interacting through weak non-covalent forces. Here, the errors of a range of GGAs for the 3-body energies of trimers of rare gases and water are investigated. The patterns of 3-body errors are similar for all the systems, and are related to the form of the exchange-enhancement factor FX(x) at large reduced gradient x, which also governs 2-body exchange-overlap errors. However, it is shown that the 3-body and 2-body errors depend in opposite ways on FX(x), so that they tend to cancel in molecular aggregates. Embedding arguments are used to achieve a partial separation of contributions to 3-body error from polarization, non-local correlation, and exchange, and it emerges that exchange is a major contributor. The practical importance of beyond-2-body errors is illustrated by the energetics of the water hexamer. An analysis of exchange-energy distributions is used to elucid...
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