Effects
of Finite Size and Proton Disorder on Lattice-Dynamics
Estimates of the Free Energy of Clathrate Hydrates
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Abstract
We
consider lattice dynamics methods for calculation of the free
energy of clathrate hydrate phases, specifically the cubic structure
I (sI), cubic structure II (sII), and hexagonal structure H (sH) phases,
in the absence of guest molecules; water molecules are modeled with
the TIP4P potential. We examine in particular the effects of finite
size and proton disorder on the calculated free energies and consider
these in the context of the free-energy differences between the phases.
We find that at 300 K, the finite-size, proton-disorder, and quantum
effects between phases are, respectively, on the order of 0.4, 0.03,
and 0.1 kJ/mol. Details of the calculations are provided, with emphasis
on efficiencies developed to handle various aspects related to rigid-molecule
lattice dynamics with electrostatic interactions in large crystalline
systems