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

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 present a stress formulation for spin vdW-DF and illustrate the use of a simple stability-modeling scheme. The modeling supplements DFT (with a specific XC functional) by asserting whether the finding of a soft mode (an imaginary-frequency vibrational mode, ubiquitous in perovskites and soft matter) implies an actual DFT-based prediction of a low-temperature transformation.

Accurate non-empirical range-separated hybrid van der Waals density functional for complex molecular problems, solids, and surfaces

“…Nonlocal-correlation functionals of the van der Waals (vdW) density functional (vdW-DF) method fulfill a necessary condition for serving as a general-purpose DFT: They incorporate the ubiquitous van der Waals (vdW) interactions that dominate intermolecular interactions. The vdW-DF method 9,[27][28][29][30][31][32][33][34][35][36][37][38][39][40][41][42][43] is a systematic way to de-sign a truly nonlocal-correlation functional from the insight that we have already gained in making semilocal functionals within the generalized gradient approximation (GGA). 9,[31][32][33][34][43][44][45][46][47][48][49][50][51][52][53] The two first method releases, termed vdW-DF1 (or vdW-DF) and vdW-DF2, have the same overall structure but differ in how a plasmon-response model sets the details of the nonlocalcorrelation energy E nl c and in choice of the exchange component.…”

“…Broader method impact and benefits as generalpurpose tools 27,36,60,61 came upon further inspection of the role of gradient-corrected exchange in the overall design for a balanced exchange-correlation (XC) energy functional design. This approach produced both variants that reflect a practice-driven analysis [60][61][62][63][64] as well as a focus on seeking nonempirical designs with an XC balance that respects constraints set by many-body perturbation theory (MBPT).…”

“…We have very recently suggested a focus on systematically exploring use of a tool chain of related vdW-DF versions. 27,42,43 This tool chain comprises the consistent-exchange regular vdW-DF-cx version 35 (abbreviated CX), the unscreened hybrid vdW-DF-cx0p 40 (here abbreviated CX0P), and the RSH vdW-DF-ahcx 42 (abbreviated AHCX). All members use the original general-geometry formulation of the nonlocal-correlation energy.…”

“…Having the smallgradient exchange form set from the early Langreth-Vosko (LV) MBPT analysis of gradient-corrections to exchange, 33,48,49 this CX-based tool chain is set up to likely succeed at bulk and surfaces 43 ; Good CX performance for molecular properties is not a priori given, although a welcome bonus. 27,42,43 The RSH AHCX is the tool-chain corner stone for it allows a mapping across both molecules and generic substrates (including metals), i.e., for the important heterogeneous-materials problems. 42,[70][71][72][73][74][75] The overall idea of using this CX-based tool chain is that we can map the interplay between truly nonlocal exchange and truly nonlocal correlation and, hopefully, thus understand how to balance them also when used in combination.…”

“…42,[70][71][72][73][74][75] The overall idea of using this CX-based tool chain is that we can map the interplay between truly nonlocal exchange and truly nonlocal correlation and, hopefully, thus understand how to balance them also when used in combination. 19,27 The present paper contains several mutually supporting analysis, design, and computational results that lead us to the formulation of a second-generation RSH vdW-DF (termed vdW-DF2-ahbr and abbreviated AHBR), as well as an associated XC tool chain. Specifically, we now start with the vdW-DF2 nonlocal-correlation energy formulation.…”