MD simulations using distributed multipole electrostatics: Structural and spectroscopic properties of CO- and methane-containing clathrates

Abstract

Molecular dynamics (MD) simulations are used to investigate the properties of empty and methane- and carbon-monoxide-containing hydrates. Intermolecular interactions are described by force fields including a combination of anharmonic bond potentials and accurate molecular electrostatics based on distributed multipoles. It is found that structural and spectroscopic properties and small differences between the systems can be correctly described with such an approach. In particular, spectra in the acoustic region (low-frequency lattice modes) delicately depend on the electrostatic interactions between the molecules for clathrates with different guests. Analysis of the radial distribution functions demonstrates that the short-range order is dominated by steric effects, whereas the long-range order is more sensitive to electrostatic interactions. Predictions are made for the acoustic modes of CO-containing clathrates and for the structural differences in the hydrogen radial distribution function between the CO- and methane-containing clathrates.

Keywords:

• intermolecular forces
• distributed multipoles
• clathrates
• atomistic simulations

Acknowledgements

The authors acknowledge financial support from the Swiss National Science Foundation and from the EU project “Molecular Universe” (to MM) and from the National Science Foundation (to DLF and JDD through award CHE0554922). We thank Professor Richard Stratt for interesting discussions.