Ab initiocalculation of the origin of the distortion of α-PbO

We present ab initio calculations using density-functional theory (DFT) within the local density approximation (LDA) on PbO in the phase and the idealized CsCl structure. The calculated structure of the phase, which is predicted to be the most stable, is in good agreement with experiment. It has a nonspherical electron distribution forming what could be described as sterically active lone pair formed by the 6s^2 electrons. However, analysis of the partial density of states reveals mixing of the Pb 6s with the oxygen 2p electronic states indicating that the classical theory of hybridization of the lead 6s and 6p orbitals is incorrect and that the ``lone pair'' is the result of the lead-oxygen interaction.