Rigid Body Approximation for the Anharmonic Description of Molecule-Surface Vibrations

Area of Science:

• Computational Chemistry
• Materials Science
• Physical Chemistry

Background:

• Accurate prediction of molecule-surface interactions is crucial for understanding adsorption phenomena.
• Traditional harmonic models often fail to capture the anharmonic nature of molecular vibrations at surfaces.
• Developing robust computational methods is essential for designing new materials with tailored adsorption properties.

Purpose of the Study:

• To develop and validate a novel anharmonic approach for calculating molecule-surface vibrations.
• To accurately determine thermodynamic functions (enthalpy, entropy, Gibbs free energy) of adsorption.
• To investigate the influence of anharmonicity and vibrational coupling on adsorption thermodynamics.

Main Methods:

• Utilized rigid body coordinates based on Rodrigues rotation formula for curvilinear displacements.
• Employed cubic splines to fit one-dimensional polynomial potentials from calculated energy data points.
• Solved one-dimensional Schrödinger equations using harmonic oscillator or Fourier basis sets for each rigid body mode.
• Incorporated coupling between molecule-surface and intrasystem vibrations at the harmonic level.

Main Results:

• Successfully tested the numerical implementation against Morse and cosine potentials with known analytical solutions.
• Applied the method to methane (CH4) adsorption on H-chabazite (H-CHA) and methane/carbon monoxide (CO) on Mg-MOF.
• Quantified the anharmonic effect on Gibbs free energy of adsorption, showing significant contributions for CH4/MOF and CO/MOF.
• Observed deviations from experimental values within chemical accuracy limits for MOF systems.

Conclusions:

• The developed anharmonic approach provides accurate predictions for adsorption thermodynamics.
• Vibrational coupling has a varying impact on adsorption, being substantial for CH4/MOF.
• The method shows promise for studying molecule-surface dynamics in zeolites and MOFs, with potential improvements for systems involving soft modes.