Eur. Phys. J. D 46, 69-76 (2008)
https://doi.org/10.1140/epjd/e2007-00285-3

Simulating CH₄ physisorption on ionic crystals

Limitations of an atomic partial charge model

Department of Chemistry, University of Tennessee, Knoxville, TN, 37996-1600, USA

Corresponding author: ^a rhinde@utk.edu

Received: 18 April 2007
Revised: 11 September 2007
Published online: 10 October 2007

Abstract

We use quantum chemical techniques to evaluate the electrostatic and polarization components of the interaction between a rigid CH₄ molecule and a lattice of point charges representing the MgO(100) surface. We find that CH₄ positioned above Mg adopts an edge-down configuration in which two H atoms are oriented downward towards the MgO(100) surface and point at O ions in the surface layer. The CH₄–MgO(100) electrostatic interaction is substantially less favorable (but is still attractive) for the face-down configuration in which three H atoms point downward. Neither configuration is energetically favorable for CH₄ molecules positioned above O ions. We show that for edge-down CH₄ molecules above Mg, the electrostatic component of the CH₄-substrate interaction varies considerably as the CH₄ molecule rotates about the surface normal; the polarization component of the interaction, by contrast, is nearly constant during this rotation. We show that a point-charge model for the CH₄ charge distribution, in which the C and H atoms carry effective partial charges, predicts that the CH₄-surface electrostatic interaction should be more favorable for face-down CH₄ molecules than for edge-down CH₄ molecules, in disagreement with the quantum chemical results. We show that this is because the point-charge model poorly represents the high-order electric multipoles of CH₄.