# How strongly do hydrogen and water molecules stick to carbon nanomaterials?

## TL;DR

This study provides benchmark interaction energies for hydrogen and water molecules with carbon nanotubes using diffusion Monte Carlo, compares various computational models, and highlights the importance of medium-range correlation in these interactions.

## Contribution

It offers the first reliable DMC benchmark data for hydrogen and water interactions with CNTs and evaluates the performance of different DFT functionals and force fields.

## Key findings

- DMC interaction energies serve as benchmarks for CNT systems.
- Some vdW DFT models poorly predict medium-range correlations.
- Certain xc functionals and force fields perform well for molecule-CNT interactions.

## Abstract

The interaction strength of molecular hydrogen and water to carbon nanomaterials is relevant to, among many applications, hydrogen storage, water treatment, and water flow. However, accurate interaction energies for hydrogen and water with carbon nanotubes(CNTs) remain scarce despite the importance of having reliable benchmark data to inform experiments and to validate computational models. Here, benchmark fixed-node diffusion Monte Carlo (DMC) interaction energies are provided for hydrogen and water monomers, inside and outside a typical zigzag CNT. The DMC interaction energies provide valuable insight into molecular interactions with CNTs in general, and are also expected to be particularly relevant to gas uptake studies on CNTs. In addition, a selection of density functional theory (DFT) exchangecorrelation(xc) functionals and force field potentials that ought to be suitable for these systems, is compared. An unexpected variation is found in the performance of DFT van der Waals (vdW) models in particular. An analysis of the peculiar discrepancy between different vdW models indicates that medium-range correlation (at circa 3 to 5 \r{A}) plays a key role inside CNTs, and is poorly predicted by some vdW models. Using accurate reference information, this work reveals which xc functionals and force fields perform well for molecules interacting with CNTs. The findings will be valuable to future work on these and related systems, that involve molecules interacting with low-dimensional systems.