First principles molecular dynamics simulation of carbon at high pressures and temperatures

The unified method for molecular dynamics and density functional theory introduced by Car and Parrinello has made possible accurate thermodynamic calculations on a wide variety of materials. The key to this method is a highly efficient means for computing the electron density as a function of the positions of the nuclei, which then leads to accurate forces on the nuclei. In this thesis we extend the original method based on finite temperature density functional theory and we present an extensive study of the phases of carbon at high temperatures and pressures. Our extension provides a general method to treat electronic states at finite temperature or in non-equilibrium excited states, alleviates convergence problems that arise when iterative methods are applied to metals, and provides a framework that allows fractional occupation numbers to vary dynamically during the simulation of metallic systems. The method is illustrated by calculations on crystalline metallic carbon and simulations of liquid silicon. Very little is known about the phase diagram of carbon at high pressures and temperatures since these conditions are currently outside the reach of experiment. We have performed a series of first principles molecular dynamics simulations which elucidate three aspects of the phase diagram: 1) the melting of the simple cubic phase at ""35 Mbar; 2) evidence for a structural change in the liquid in the range of 3-8 Mbar; and 3) the melting of the BC-8 phase at ""20 Mbar. We combine the results of these three investigations to construct a proposed phase diagram for carbon at high temperatures and pressures.U of I OnlyThesi