The energy of coincidence twist boundaries formed by rotation about a  axis in MgO crystals has been calculated using the Born-Mayer potential and rigid lattice approximation. It is found that the four prominent coincidence twist boundaries at 16.26, 22.62, 28.07, and 36.87° have local energy minima. The coincidence boundary formed by a rotation of 28.07° has a positive contribution to energy from the electrostatic term and therefore has the shallowest minima. This is in agreement with experimental observations on crystals of MgO smoke. © 1972.