"Plug-and-Play" potentials: Investigating quantum effects in (H2)2-Li+-benzene.

Quantum and anharmonic effects are investigated in (H2)2-Li+-benzene, a model for hydrogen adsorption in metal-organic frameworks and carbon-based materials, using rigid-body diffusion Monte Carlo (RBDMC) simulations. The potential-energy surface (PES) is calculated as a modified Shepard interpolation of M05-2X/6-311+G(2df,p) electronic structure data. The RBDMC simulations yield zero-point energies (ZPE) and probability density histograms that describe the groundstate nuclear wavefunction. Binding a second H2 molecule to the H₂-Li⁺-benzene complex increases the ZPE of the system by 5.6 kJ mol^-1 to 17.6 kJ mol^-1. This ZPE is 42% of the total electronic binding energy of (H₂)₂-Li⁺-benzene and cannot be neglected. Our best estimate of the 0 K binding enthalpy of the second H₂ to H₂-Li⁺-benzene is 7.7 kJ mol^-1, compared to 12.4 kJ mol^-1 for the first H₂ molecule. Anharmonicity is found to be even more important when a second (and subsequent) H₂ molecule is adsorbed; use of harmonic ZPEs results in significant error in the 0 K binding enthalpy. Probability density histograms reveal that the two H2 molecules are found at larger distance from the Li⁺ ion and are more confined in the coordinate than in H₂-Li⁺-benzene. They also show that both H₂ molecules are delocalized in the azimuthal coordinate, f. That is, adding a second H₂ molecule is insufficient to localize the wavefunction in f. Two fragment-based (H₂)₂-Li⁺-benzene PESs are developed. These use a modified Shepard interpolation for the Li+-benzene and H₂-Li⁺-benzene fragments, and either modified Shepard interpolation or a cubic spline to model the H₂-H₂ interaction. Because of the neglect of three-body H₂, H₂, Li⁺ terms, both fragment PESs lead to overbinding of the second H₂ molecule by 1.5 kJ mol^-1. Probability density histograms, however, indicate that the wavefunctions for the two H₂ molecules are effectively identical on the "full" and fragment PESs. This suggests that the 1.5 kJ mol^-1 error is systematic over the regions of configuration space explored by our simulations. Notwithstanding this, modified Shepard interpolation of the weak H2-H2 interaction is problematic and we obtain more accurate results, at considerably lower computational cost, using a cubic spline interpolation. Indeed, the ZPE of the fragment-with-spline PES is identical, within error, to the ZPE of the full PES. This fragmentation scheme therefore provides an accurate and inexpensive method to study higher hydrogen loading in this and similar systems. [ABSTRACT FROM AUTHOR]