  We discuss the structural and electronic properties of tetragonal CuO grown
on SrTiO3(100) by means of hybrid density functional theory. Our analysis
explains the anomalously large Cu-O vertical distance observed in the
experiments (~2.7 A) in terms of a peculiar frustration between two competing
local Cu-O environments characterized by different in-plane and out-of-plane
bond lengths and Cu electronic populations. The proper inclusion of substrate
effects is crucial to understand the tetragonal expansion and to reproduce
correctly the measured valence band spectrum for a CuO thickness of 3-3.5 unit
cells, in agreement with the experimentally estimated thickness.
