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Molecular hydrogen is at the core of hydrogen energy applications and has thepotential to significantly reduce the use of carbon dioxide emitting energyprocesses. However, hydrogen gas storage is a major bottleneck for itslarge-scale use as current storage methods are energy intensive. Amongdifferent storage methods, physisorbing molecular hydrogen at ambient pressureand temperatures is a promising alternative - particularly thanks to tuneablelightweight nanomaterials and high throughput screening methods. Nonetheless,understanding hydrogen adsorption in well-defined nanomaterials remainsexperimentally challenging and reference information is scarce despite theproliferation of works predicting hydrogen adsorption. In this work, we focuson Li, Na, Ca, and K, decorated graphene sheets as substrates for hydrogenadsorption and compute the most accurate adsorption energies available to dateusing quantum diffusion Monte Carlo (DMC). Building on our previous insights atthe density functional theory (DFT) level, we find that a weak covalentchemisorption of molecular hydrogen, known as Kubas binding, is feasible on Cadecorated graphene according to DMC, in agreement with DFT. This finding is incontrast to previous DMC predictions of the 4H$_2$/Ca$^+$ gas cluster wherechemisorption is not favoured. However, we find that the adsorption energy ofhydrogen on metal decorated graphene according to a widely-used DFT method isnot fully consistent with DMC and the discrepancies are not systematic. Thereference adsorption energies reported herein can be used to find betterwork-horse methods for application in large-scale modelling of hydrogenadsorption. Furthermore, the implications of this work affect strategies forfinding suitable hydrogen storage materials and high-throughput methods.

Unravelling H$_2$ chemisorption and physisorption on metal decorated graphene using quantum Monte Carlo