A path-integral molecular dynamics simulation of diamond

Diamond is studied by path integral molecular dynamics simulations of the atomic nuclei in combination with a tight-binding Hamiltonian to describe its electronic structure and total energy. This approach allows us to quantify the influence of quantum zero-point vibrations and finite temperatures on both the electronic and vibrational properties of diamond. The electron-phonon coupling mediated by the zero-point vibration reduces the direct electronic gap of diamond by 10 %. The calculated decrease of the direct gap with temperature shows good agreement with the experimental data available up to 700 K. Anharmonic vibrational frequencies of the crystal have been obtained from a linear-response approach based on the path integral formalism. In particular, the temperature dependence of the zone-center optical phonon has been derived from the simulations. The anharmonicity of the interatomic potential produces a red shift of this phonon frequency.At temperatures above 500 K, this shift is overestimated in comparison to available experimental data. The predicted temperature shift of the elastic constant c_{44} displays reasonable agreement with the available experimental results.