# Hydrogen bonding characterization in water and small molecules

## TL;DR

This paper uses first-principles Density Functional Theory calculations to analyze hydrogen bonds in water and small molecules, revealing detailed electronic and structural characteristics and the influence of cooperative effects.

## Contribution

It introduces a novel geometric parameter derived from Wannier functions that correlates with bond order and covalency in hydrogen bonds.

## Key findings

- Hydrogen bonds show a range of electron density distributions and shifts in Wannier centers.
- The new geometric parameter correlates with the Mayer bond order and estimates covalent contribution.
- Cooperative effects significantly influence hydrogen-bond formation in water clusters and ice.

## Abstract

The prototypical Hydrogen bond in water dimer and Hydrogen bonds in the protonated water dimer, in other small molecules, in water cyclic clusters, and in ice, covering a wide range of bond strengths, are theoretically investigated by first-principles calculations based on the Density Functional Theory, considering a standard Generalized Gradient Approximation functional but also, for the water dimer, hybrid and van-der-Waals corrected functionals. We compute structural, energetic, and electrostatic (induced molecular dipole moments) properties. In particular, Hydrogen bonds are characterized in terms of differential electron densities distributions and profiles, and of the shifts of the centres of Maximally localized Wannier Functions. The information from the latter quantities can be conveyed into a single geometric bonding parameter that appears to be correlated to the Mayer bond order parameter and can be taken as an estimate of the covalent contribution to the Hydrogen bond. By considering the cyclic water hexamer and the hexagonal phase of ice we also elucidate the importance of cooperative/anticooperative effects in Hydrogen-bonding formation.