Another peculiar feature of the P2 1 /c structure of CuP 2 is that pairs of Cu atoms are quite close to each other (2.48 Å). Comparing this distance to the metallic and covalent radii of Cu suggests that some metallic Cu-Cu bonding is to be expected, which is confirmed by calculation of the electron localization function. These Cu-Cu dimers were recently shown to vibrate anharmonically as a rattling mode and strongly scatter acoustic phonons. This is the key feature enabling low lattice thermal conductivity in CuP 2 in spite of its relatively high acoustic conductivity, thus making it interesting for thermoelectrics.