**Generalized ensemble method applied to study systems with strong first
order transitions**

E Malolepsza and J Kim and T Keyes, XXVI IUPAP CONFERENCE ON COMPUTATIONAL PHYSICS (CCP2014), 640, 012003 (2015).

DOI: 10.1088/1742-6596/640/1/012003

At strong first-order phase transitions, the entropy versus energy or,
at constant pressure, enthalpy, exhibits convex behavior, and the
statistical temperature curve correspondingly exhibits an S-loop or
back-bending. In the canonical and isothermal-isobaric ensembles, with
temperature as the control variable, the probability density functions
become bimodal with peaks localized outside of the S-loop region.
Inside, states are unstable, and as a result simulation of equilibrium
phase coexistence becomes impossible. To overcome this problem, a method
was proposed by Kim, Keyes and Straub **1**, where optimally designed
generalized ensemble sampling was combined with replica exchange, and
denoted generalized replica exchange method (gREM). This new technique
uses parametrized effective sampling weights that lead to a unimodal
energy distribution, transforming unstable states into stable ones. In
the present study, the gREM, originally developed as a Monte Carlo
algorithm, was implemented to work with molecular dynamics in an
isobaric ensemble and coded into LAMMPS, a highly optimized open source
molecular simulation package. The method is illustrated in a study of
the very strong solid/liquid transition in water.

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