Orbital densities functional

TL;DR

This paper introduces the orbital densities functional (ODF), a new approach that corrects the derivative discontinuity in density functional theory, improves energy gap predictions, and accurately models the metal-insulator transition in lanthanum trihydride.

Contribution

The paper proposes the orbital densities functional (ODF) with Wannier orbitals that enforces the discontinuity condition and improves the description of energy gaps and correlation effects.

Findings

ODF corrects the energy gap underestimation of LDA.

ODF predicts insulating states in LaH_{3-x} where LDA predicts metallic.

Application of ODF with DMFT captures the metal-insulator transition accurately.

Abstract

Local density approximation (LDA) to the density functional theory (DFT) has continuous derivative of total energy as a number of electrons function and continuous exchange-correlation potential, while in exact DFT both should be discontinuous as number of electrons goes through an integer value. We propose orbital densities functional (ODF) (with orbitals defined as Wannier functions) that by construction obeys this discontinuity condition. By its variation one-electron equations are obtained with potential in the form of projection operator. The operator increases a separation between occupied and empty bands thus curing LDA deficiency of energy gap value systematic underestimation. Orbital densities functional minimization gives ground state orbital and total electron densities. The ODF expression for the energy of orbital densities fluctuations around the ground state values defines ODF fluctuation Hamiltonian that allows to treat correlation effects. Dynamical mean-field theory (DMFT) was used to solve this Hamiltonian with quantum Monte Carlo (QMC) method for effective impurity problem. We have applied ODF method to the problem of metal-insulator transition in lanthanum trihydride LaH_{3-x}. In LDA calculations ground state of this material is metallic for all values of hydrogen nonstoichiometry x while experimentally the system is insulating for x < 0.3. ODF method gave paramagnetic insulator solution for LaH_3 and LaH_{2.75} but metallic state for LaH_{2.5}.