# The quantum nature of hydrogen

## TL;DR

This paper reviews recent advances in understanding the quantum behavior of hydrogen nuclei and their significant impact on material properties, phase boundaries, surface reactions, and biological hydrogen bonds.

## Contribution

It highlights methodological developments enabling accurate simulation of nuclear quantum effects in complex hydrogen-containing systems.

## Key findings

- Quantum effects lower hydrogen melting lines.
- Nuclear quantum effects facilitate hydrogen dissociation and diffusion.
- Quantum effects alter hydrogen bond strength and structure.

## Abstract

Hydrogen is the most abundant element in the universe. It is also the lightest and as such the most quantum of the elements, in the sense that quantum tunnelling, quantum delocalisation, and zero-point motion can be important. For practical reasons most computer simulations of materials have not taken such effects into account, rather they have treated atomic nuclei as classical point-like particles. However, it is an exciting time for the theory and simulation of materials and thanks to significant methodological developments over the last few decades, nuclear quantum effects can now be accurately treated in complex materials. In this brief review we discuss our recent studies on the role nuclear quantum effects play in hydrogen containing systems. We give examples of how the quantum nature of the nuclei has a significant impact on the location of the boundaries between phases in high pressure condensed hydrogen, including a dramatic lowering of the solid to liquid melting line. We show how nuclear quantum effects facilitate the dissociative adsorption of molecular hydrogen on solid surfaces and the diffusion of atomic hydrogen across surfaces; effects that are of relevance to the catalytic performance of the surfaces. Finally, we discuss how nuclear quantum effects alter the strength and structure of hydrogen bonds, including the hydrogen bonds in DNA. Overall these studies demonstrate that nuclear quantum effects can manifest in many different, interesting, and at times non-intuitive ways. Whilst historically it has been difficult to know in advance what influence nuclear quantum effects will have, some of the important conceptual foundations have now started to emerge.