With the use of first principles theory we confirm the semiconducting ground state of the technologically interestingYH3compound. Thus, unlike previously published theories, we demonstrate that density functional theory in the local density approximation reproduces the existence of a band gap. We also show that theHoD3structure is the stable structure at ambient conditions, in agreement with experiment. In addition, we predict that moderate applied pressure will driveYH3into the cubic structure. ©1997 American Institute of Physics.