# Molecular dynamics simulation of particle trajectory for the evaluation of surface accommodation coefficients

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## Abstract

Molecular dynamics approach has been employed in this work to model experiments for the interaction of a single particle with a surface. Some important modifications are implemented in the present approach as compared to the previous works. Investigations have been performed to compute the reflected distribution of molecules and calculate accommodation coefficients for interaction of noble gases with platinum and graphite surfaces. Through a few numerical tests, it is verified that the results obtained using the present approach are in good agreement with some established data in the literature. The method is then utilized to predict the surface accommodation coefficients, which are required as boundary conditions in the gas–surface interaction models, such as the Cercignani–Lampis–Lord (CLL) model, for mesoscopic simulations using the direct simulation Monte Carlo approach. Interestingly, it is shown that the CLL model shows some disagreement with the molecular dynamics simulation data for the distribution of reflected molecules. However, the CLL model can still be utilized in practical applications, as it is shown to predict the average macroscopic properties to a very high degree of accuracy. The behavior of accommodation coefficients with a change in the incident flow properties, such as bulk velocity, gas temperature, and gas-to-surface molecule mass ratio, is also investigated in detail.

## Nomenclature

- \(\lambda\)
Mean free path

*L*Characteristic length

*Kn*Knudsen number

- \(\Phi\)
Flux of a physical property

*M*Mass of the molecule

*T*Temperature

- \(k_\text {B}\)
Boltzmann constant

- \(\sigma _\text {t}\)
Tangential momentum accommodation coefficient

- \(\alpha\)
Energy accommodation coefficient

- \(\sigma\)
Size parameter for LJ potential

- \(\varepsilon\)
Depth of the potential well

*a*Lattice constant

- \(r_{0,\text {cc}}\)
Equilibrium bond length

- \(\theta _{0,\text {cc}}\)
Equilibrium angle for graphite

*k*Harmonic style parameter

- \(\phi\)
Azimuthal angle

- \(\theta\)
Elevation angle

*c*Speed of the molecule

## Subscripts

- n
Normal to the surface (

*z*-direction)- i
Incident

- r
Reflected

- s
Surface

- mp
Most probable speed

*t*1,*t*2Tangential direction to the surface;

*x*- and*y*-directions respectively- g
Gas

- gs
Gas–surface parameter

- bond
Bond parameters

- angle
Angle parameters

- dih
Dihedral angle parameters

## Notes

### Acknowledgements

The authors acknowledge the use of high-performance computing facilities available at the Indian Institute of Technology Kanpur, without which this research work would not have been possible.

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