Water on BN doped benzene: a hard test for exchange-correlation functionals and the impact of exact exchange on weak binding.

Yasmine S Al-Hamdani,O Anatole Von Lilienfeld,Angelos Michaelides,Dario Alfè

doi:10.1063/1.4898356

Abstract

Density functional theory (DFT) studies of weakly interacting complexes have recently focused on the importance of van der Waals dispersion forces, whereas the role of exchange has received far less attention. Here, by exploiting the subtle binding between water and a boron and nitrogen doped benzene derivative (1,2-azaborine) we show how exact exchange can alter the binding conformation within a complex. Benchmark values have been calculated for three orientations of the water monomer on 1,2-azaborine from explicitly correlated quantum chemical methods, and we have also used diffusion quantum Monte Carlo. For a host of popular DFT exchange-correlation functionals we show that the lack of exact exchange leads to the wrong lowest energy orientation of water on 1,2-azaborine. As such, we suggest that a high proportion of exact exchange and the associated improvement in the electronic structure could be needed for the accurate prediction of physisorption sites on doped surfaces and in complex organic molecules. Meanwhile to predict correct absolute interaction energies an accurate description of exchange needs to be augmented by dispersion inclusive functionals, and certain non-local van der Waals functionals (optB88- and optB86b-vdW) perform very well for absolute interaction energies. Through a comparison with water on benzene and borazine (B3N3H6) we show that these results could have implications for the interaction of water with doped graphene surfaces, and suggest a possible way of tuning the interaction energy.

Highlights

An accurate description of the structures and energies of weakly interacting systems is important in materials science and biology, but it is often difficult to obtain reference data either theoretically or experimentally
A key challenge lies in the ability to capture small energy differences – on the order of a few meV – that can have drastic effects on the structure
In biological applications there can be numerous shallow energy minima with different conformations

Summary

Introduction

An accurate description of the structures and energies of weakly interacting systems is important in materials science and biology, but it is often difficult to obtain reference data either theoretically or experimentally. A key challenge lies in the ability to capture small energy differences – on the order of a few meV – that can have drastic effects on the structure. Water has several distinct ice polymorphs that have lattice energies within 35 meV/H2O of each other.. Likewise for water clusters, most notably the water hexamer, there are several isomers that have energies within just 5 meV/H2O.5,7,8. In biological applications there can be numerous shallow energy minima with different conformations. For complex organic systems, predicting the exact lowest energy conformation is crucial to determine the crystal structure of drugs and the mechanisms by which proteins function.

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Conclusion