Partition-DFT on the Water Dimer

TL;DR

This paper applies Partition Density Functional Theory to analyze the water dimer's electronic structure across different O-O distances and symmetries, providing insights into hydrogen-bond formation.

Contribution

It introduces a PDFT-based approach to decompose water dimer densities across a range of geometries, highlighting the energetic and electronic differences between symmetries.

Findings

Work to deform monomer densities is slightly larger for $C_s$ geometry.

PDFT densities support the orbital-interaction picture of hydrogen bonds.

Partition potentials reveal electronic structure changes with O-O distance.

Abstract

As is well known, the ground-state symmetry group of the water dimer switches from its equilibrium $C_{s}$-character to $C_{2h}$-character as the distance between the two oxygen atoms of the dimer decreases below $R_{\rm O-O}\sim 2.5$ \AA{}. For a range of $R_{\rm O-O}$ between 1 and 5 \AA{}, and for both symmetries, we apply Partition Density Functional Theory (PDFT) to find the unique monomer densities that sum to the correct dimer densities while minimizing the sum of the monomer energies. We calculate the work involved in deforming the isolated monomer densities and find that it is slightly larger for the $C_s$ geometry for all $R_{\rm O-O}$. We discuss how the PDFT densities and the corresponding partition potentials support the orbital-interaction picture of hydrogen-bond formation.