**Simulations of dipolar fluids using effective many-body isotropic
interactions**

JO Sindt and PJ Camp, JOURNAL OF CHEMICAL PHYSICS, 143, 024501 (2015).

DOI: 10.1063/1.4923300

The partition function of a system with pairwise-additive anisotropic
dipole-dipole interactions is equal to that of a hypothetical system
with many-body isotropic interactions **G. Stell, Phys. Rev. Lett. 32,
286 (1974)**. The effective many-body interactions contain n-body
contributions of all orders. Each contribution is known as an expansion
in terms of the particle-particle distances r, and the coefficients are
temperature dependent. The leading-order two-body term is the familiar
-r(-6) attraction, and the leading-order three-body term is equivalent
to the Axilrod-Teller interaction. In this work, a fluid of particles
with the leading-order two-body and three-body interactions is compared
to an equivalent dipolar soft-sphere fluid. Molecular simulations are
used to determine the conditions under which the effective many-body
interactions reproduce the fluid-phase structures of the dipolar system.
The effective many-body interaction works well at moderately high
temperatures but fails at low temperatures where particle chaining is
expected to occur. It is shown that an adjustment of the coefficients of
the two-body and three-body terms leads to a good description of the
structure of the dipolar fluid even in the chaining regime, due
primarily to the ground-state linear configuration of the three-body
Axilrod-Teller interaction. The vapor-liquid phase diagrams of systems
with different Axilrod-Teller contributions are determined. As the
strength of the three-body interaction is increased, the critical
temperature and density both decrease and disappear completely above a
threshold strength, where chaining eventually suppresses the
condensation transition. (C) 2015 AIP Publishing LLC.

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