Hard and soft materials: Putting consistent van der Waals density functionals to work

TL;DR

This paper develops a tool chain of van der Waals density functionals to better understand when nonlocal exchange and correlation are crucial, applying it to materials like perovskites, biomolecules, and ferroelectric polymers.

Contribution

It introduces a consistent framework of related hybrid exchange-correlation functionals within vdW-DF, enabling systematic mapping of nonlocal effects in complex materials.

Findings

Identified when nonlocal exchange and correlation are essential in materials.

Predicted structure and polarization of a ferroelectric polymer.

Developed a stress formulation for spin vdW-DF and a stability model for soft modes.

Abstract

We present the idea and illustrate potential benefits of having a tool chain of closely related regular, unscreened and screened hybrid exchange-correlation (XC) functionals, all within the consistent formulation of the van der Waals density functional (vdW-DF) method [JPCM 32, 393001 (2020)]. Use of this chain of nonempirical XC functionals allows us to map when the inclusion of truly nonlocal exchange and of truly nonlocal correlation is important. Here we begin the mapping by addressing hard and soft material challenges: magnetic elements, perovskites, and biomolecular problems. We also predict the structure and polarization for a ferroelectric polymer. To facilitate this work and future broader explorations, we furthermore present a stress formulation for spin vdW-DF and illustrate use of a simple stability-modeling scheme to assert when the prediction of a soft mode (an imaginary-frequency vibrational mode, ubiquitous in perovskites and soft matter) implies a prediction of an actual low-temperature transformation.