Correcting density-driven errors in projection-based embedding.

Robert C R Pennifold,Frederick R Manby,Simon J Bennie,Thomas F Miller

doi:10.1063/1.4974929

Robert C R Pennifold, Frederick R Manby + Show 2 more

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https://doi.org/10.1063/1.4974929

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Abstract

Projection-based embedding provides a simple and numerically robust framework for multiscale wavefunction-in-density-functional-theory (WF-in-DFT) calculations. The approach works well when the approximate DFT is sufficiently accurate to describe the energetics of the low-level subsystem and the coupling between subsystems. It is also necessary that the low-level DFT produces a qualitatively reasonable description of the total density, and in this work, we study model systems where delocalization error prevents this from being the case. We find substantial errors in embedding calculations on open-shell doublet systems in which self-interaction errors cause spurious delocalization of the singly occupied orbital. We propose a solution to this error by evaluating the DFT energy using a more accurate self-consistent density, such as that of Hartree-Fock (HF) theory. These so-called WF-in-(HF-DFT) calculations show excellent convergence towards full-system wavefunction calculations.

Highlights

Density-functional theory (DFT) is an extremely powerful tool for exploring a range of chemical problems, including reactivity,1–3 molecular spectroscopy,4,5 the study of inorganic crystals,6 enzyme catalysis7 and simulations of materials,8 amongst many others
It is necessary that the low-level DFT produces a qualitatively reasonable description of the total density, and in this work, we study model systems where delocalization error prevents this from being the case
Projection-based embedding30 is an effective method for performing wavefunction-in-density-functional-theory (WFin-DFT) calculations, and even when subsystem A is small, coupled-cluster-type accuracy is typically approached

Summary

INTRODUCTION

Density-functional theory (DFT) is an extremely powerful tool for exploring a range of chemical problems, including reactivity, molecular spectroscopy, the study of inorganic crystals, enzyme catalysis and simulations of materials, amongst many others. Main sources of error in projection-based error have been analysed by Goodpaster et al, who found that in cases where embedding performs less well, the error is often dominated by the non-additive exchange-correlation energy.47 Other errors such as those that occur from the use of DFT on subsystem B, or in the embedding potential, were observed to be relatively minor for the systems studied by Goodpaster et al.. We investigate cases where large errors occur for CCSD(T)-in-DFT embedding but not for CCSD(T)-in-HF We show that these errors arise from an inaccurate description of the density by DFT, which originate from the incomplete cancellation of the self-interaction component of the Coulomb and exchange-correlation energies. This could be of particular importance for calculations on transition metal clusters, which often have a large number of unpaired electrons

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Findings

CONCLUSIONS