Paper title:

Thermal conductivity, shear viscosity and specific heat of rigid water models

 

Journal:

Chemical Physics Letters 542 (2012) 37?41

 

Author/s:

Yijin Mao, Yuwen Zhang

Department of Mechanical and Aerospace Engineering, University of Missouri, Columbia, MO 65211, USA

 

 

Summary:

In this paper, reverse nonequilibrium molecular dynamics (RNEMD) was used to determine the thermal conductivity, shear viscosity, and specific heat of 8 water models, namely: SPC, SPC/E, TIP3P, TIP4P, TIP4P-2005, TIP4P-Ew, TIP5P, and TIP5P-Ew.

A lot of water models were developed because, as of the moment, no single model can perfectly predict all properties of water yet. Some of them will have strengths in one aspect but weaknesses in others. In this study, the authors investigated 3-site (SPC, SPC/E, TIP3P), 4-site (TIP4P, TIP4P-2005, TIP4P-Ew), and 5-site (TIP5P, TIP5P-Ew) water models to determine which among them is best for determining thermal conductivity, shear viscosity, and specific heat of water.

RNEMD is deemed to be a method similar to experimental techniques used in laboratories to measure thermal and dynamic properties of fluids. Using RNEMD with NVT and NPT ensembles, simulations were conducted using a pressure of 1 atm while the temperatures were controlled for different runs (range: 283 K to 343 K), depending on the need for calculating a property. The three properties were calculated as follows:

Thermal conductivity was calculated based on Fourier’s Law (using heat flux and temperature gradient). Heat flux was determined through the accumulated molecular kinetic energies exchanged between the slabs while temperature gradient was calculated from the average temperature of all atoms within each slab.

Shear viscosity was calculated by averaging all atomic velocities. Each MD simulation for calculating this property ends when there is symmetric temperature or when the velocity profile is already established.

Calculation of the specific heat involves determination of the enthalpy of the system. This is done by using the thermodynamic relation between enthalpy, internal energy, pressure, and volume (H=U+PV).

It was concluded from the simulation results that the 5-site models can give the most accurate thermal conductivity among the water models studied but TIP5P-Ew is the best model for predicting the trend of thermal conductivity. Moreover, the 5-site models can accurately predict shear viscosity and its trends along with temperature. But for specific heat, the best model to use is the 3-site model, specially TIP3P.

 

Contribution and application:

This paper presents a detailed procedure for calculating thermal conductivity, shear viscosity, and specific heat in a molecular dynamics simulation. The authors also gave the governing principles and equations for the calculation of the 3 properties. These calculation procedures might be useful for our lab’s own MD studies. In addition, the paper can also be used as basis for selecting which water model can be used for our simulations, which will depend on what fluid properties we are interested in.

 

By: Hannah Ebro

hannah@gist.ac.kr