Ab initio molecular dynamics with a finite-temperature density functional

The unified method for molecular dynamics and density functional theory (MD/DF) introduced by Car and Parrinello is based on zero-temperature density functional theory. We have incorporated the finite-temperature extension of density functional theory proposed by Mermin into a consistent fictitious Lagrangian framework. Such an extension of the original MD/DF method is desirable for two rather different reasons. First this framework provides a general method to treat electronic states at finite temperature or in non-equilibrium excited states. Second it can alleviate certain practical problems that arise when Kohn-Sham DF methods in general and MD/DF in particular are applied to metallic and near-metallic systems. Our approach involves dynamically varying occupation numbers which is important for states near the Fermi energy. We show that the added degrees of freedom of these states can be used to accelerate the convergence to the electronic ground state. In MD simulations this improved response of the electrons also leads to an increase in the rate of energy transfer from the ionic to the electronic degrees of freedom. Our method is illustrated by calculations on crystalline metallic carbon and simulations of liquid silicon.