<i>Ab initio</i>calculation 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 \ensuremath{\alpha} phase and the idealized CsCl structure. The calculated structure of the \ensuremath{\alpha} 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.