Constant pressure first-principles molecular dynamics study on BN, AlN, and GaN

We have performed first-principles total energy calculations to obtain various physical parameters for both zinc-blende and wurtzite BN, AlN, and GaN. The calculation has been done within the local density approximation to the density functional theory together with plane wave expansion and norm-conserving pseudopotentials. Constant-pressure first-principles molecular dynamics method has been used to relax the unit cell shape and the atomic geometry. In calculating the elastic constants and the deformation potentials, the internal displacement of atoms in the strained crystal is simulated by moving atoms with first-principles molecular dynamics method. The optical phonon frequencies at the Brillouin zone center except for LO phonons are calculated within harmonic approximation by calculating the forces acting on atoms which are displaced from equilibrium positions according to the symmetry of each mode.