Extending the Density Functional Tight Binding Method to Carbon Under Extreme Conditions

We report herein on simulations of carbon under pressures up to 2000 GPa and 30 000 K using the density functional tight binding method (DFTB) with a parameter set we have specifically designed for these conditions. The DFTB method can provide a high throughput simulation capability compared to Kohn-Sham density functional theory while retaining most of its accuracy. We fit the DFTB repulsive energy to measured and computed diamond isothermal compression data and show that this yields accurate compression curves for diamond, graphite, and the BC8 phase, as well as material properties for all three phases. We then show that our new repulsive energy yields predictions of the Hugoniot of diamond shock compressed to the conducting liquid that are within the range of different experimental measurements. Our results provide a straightforward method by which DFTB can be extended to studies of covalently bonded materials under extremely high pressures and temperatures such as the interiors of planets and other large celestial bodies.