Nature of the insulating state inLaH3

This paper reports a combined first-principles and many-body study of the nature of the insulating state in LaH${}_3$, focusing on the role of the electron correlation, which has been suggested as the mechanism responsible for the observed insulating behavior. The first-principles local-density-approximation (LDA) calculation produces a correct lattice constant but results in a semimetallic ground state, contrary to a recent optical experiment that showed that LaH${}_3$ is an insulator with a gap of 1.8 eV. An extended Hubbard model is constructed for LaH${}_3$ with its parameters extracted from the results of LDA total energy calculations. An exact-diagonalization study is then carried out to study the effect of the many-body correlation. The results show that the model produces an insignificant reduction of band overlap, thus demonstrating that the insulating ground state of LaH${}_3$ is not a Hubbard-type correlation-driven insulator.