### Calculation of heat capacities of light and heavy water by path integral
molecular dynamics

Motoyuki Shiga and __Wataru Shinoda__

J. Chem. Phys. **123** 134502 (2005).

As an application of atomistic simulation methods to heat capacities, path
integral molecular dynamics has been used to calculate the constant-volume
heat capacities of light and heavy water in the gas, liquid, and solid
phases. While the classical simulations based on conventional molecular
dynamics has estimated the heat capacities too high, the quantum simulation
based on path integral molecular dynamics has given reasonable results
based on the simple point charge/flexible potential model. The calculated
heat capacities (divided by the Boltzmann constant) in the quantum simulation
are 3.1 in the vapor H_{2}O at 300 K, 6.9 in the liquid H_{2}O at 300 K, and
4.1 in the ice I_{h} H_{2}O at 250 K, respectively, which are comparable to the experimental data
3.04, 8.9, and 4.1, respectively. The quantum simulation also reproduces
the isotope effect. The heat capacity in the liquid D_{2}O has been calculated to be 10% higher than that of H_{2}O, while it is 13% higher in experiment. The results demonstrate that the
path integral simulation is a promising approach to quantitatively evaluate
the heat capacities for molecular systems, taking account of quantum mechanical
vibrations as well as strongly anharmonic motions.

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