Molecular hydrogen confined within nanoporous framework materials: Comparison of density functional and classical force-field descriptions

The effect of confinement on the energetics, structure, and absorption of molecular hydrogen is calculated via systematically increasing the H₂ loading in the relatively inert nanoporous siliceous material sodalite (SOD). Treatments of both the H₂-H₂ and H₂-SOD interactions by both periodic density functional theory (DFT) employing four different functionals (LDA, PW91, PBE, and BLYP) and by two accurately parameterized force-field (FF) sets are critically compared. We find for all loadings of H₂ molecules the results differ significantly depending on the method employed. Through a detailed analysis of the H₂-H₂ and H₂-SOD interactions in each case we assess the performance of each method employed. We find that none of the tested functionals appear to give a good overall description of our confined H₂ cluster system and the use of well-parameterized FFs is recommended for obtaining a reasonable physical description of such systems.