Parameter-free calculation of single-particle electronic excitations inYH3

In recent optical experiments for ${\mathrm{YH}}_3,$ a large band gap of almost 3 eV has been observed. In contrast, calculations within the local density approximation (LDA) result in a semimetallic band structure with a band overlap near the Fermi level of more than 1 eV. This unusually large discrepancy between the LDA results and experiment has led to suggestions that strong local correlation effects, similar to those found in transition metal oxides, are important in ${\mathrm{YH}}_3.$ Here we conclude from parameter-free quasi-particle calculations within the GW approximation that ${\mathrm{YH}}_3$ is essentially a conventional semiconductor. We argue that the experimental results can be fully understood in band structure terms without invoking strong local correlations. The unusually large error in the band structure made by LDA is traced to its poor description of the electronic structure of the hydrogen atom. Our GW results predict a fundamental band gap of only 1 eV and an optical gap of 2.6 eV, the difference being due to vanishing matrix elements for optical transitions at lower energies. Our prediction of a small fundamental gap could be experimentally confirmed by a combination of photoemission and inverse photoemission experiments.