Exact quantum dynamics background of dispersion interactions: case study for CH${}_4\cdot$Ar in full (12) dimensions

Abstract

A full-dimensional \emph{ab initio} potential energy surface of spectroscopic quality is developed for the van-der-Waals complex of a methane molecule and an argon atom. Variational vibrational states are computed on this surface including all twelve (12) vibrational degrees of freedom of the methane-argon complex using the GENIUSH computer program and the Smolyak sparse grid method. The full-dimensional computations make it possible to study fine details of the interaction and distortion effects and to make a direct assessment of the reduced-dimensionality models often used in the quantum dynamics study of weakly-bound complexes. A 12-dimensional (12D) vibrational computation including only a single harmonic oscillator basis function (9D) to describe the methane fragment (for which we use the ground-state effective structure as the reference structure) has a 0.40 cm${}^{-1}$ root-mean-square error (rms) with respect to the converged 12D bound-state excitation energies, which is less than half of the rms of the 3D model set up with the $⟨r{⟩}_0$ methane structure. Allowing 10 basis functions for the methane fragment, the rms of the bound state vibrational energies is reduced to 0.07 cm${}^{-1}$, which is much better than the 3D models. The full-dimensional potential energy surface correctly describes the dissociation of the system, which together with further development of the variational (ro)vibrational methodology opens the route for the study of the role of dispersion forces on the excited methane vibrations and the energy transfer from the intra- to the intermolecular vibrational modes.

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• A Smolyak algorithm adapted to a system-bath separation: application to an encapsulated molecule with large amplitude motions
• Fingerprint region of the formic acid dimer: variational vibrational computations in curvilinear coordinates
• Vibrational infrared and Raman spectrum of HCOOH from variational computations
• Variational Vibrational States of Methanol (12D)
• CH${}_4\cdot$F${}^{-}$ revisited: Full-dimensional ab initio potential energy surface and variational vibrational states
• Exact quantum dynamics of methanol: full-dimensional ab initio potential energy surface of spectroscopic quality and variational vibrational states
• Methane dimer rovibrational states and Raman transition moments
• Calculating potential energy surfaces with quantum computers by measuring only the density along adiabatic transitions