First-principles molecular-dynamics simulation of liquid and amorphous selenium

A molecular-dynamics simulation has been carried out for liquid and amorphous selenium in the temperature range 350--720 K. The simulation used 64 atoms in a constant-volume simple cubic cell, and the energy surfaces and forces were calculated using the density-functional formalism, with a local-density approximation for the exchange-correlation energy. The main focus is on the microscopic atomic structure, with the associated short- and intermediate-range ordering phenomena. Ring statistics, pair correlation functions g(r), structure factors S(Q), bond angle and dihedral angle distribution functions, phonon densities of states, and diffusion constants D were computed and compared with experimental and theoretical data where available. Several structural models proposed previously for Se are evaluated and their predictions compared with the results of the present simulation. Special emphasis is given to the concentration, structure, and stability of the dominant defects, and a new structure and defect model is presented.