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A new method is presented for performing first-principles molecular-dynamics simulations of systems with variable occupancies. We adopt a matrix representation for the one-particle statistical operator ^ to introduce a ``projected'' free energy functional G that depends on the Kohn-Sham orbitals only and that is invariant under their unitary transformations. The Liouville equation ^, ^H0ex{0ex}=0ex{0ex}0 is always satisfied, guaranteeing a very efficient and robust variational minimization algorithm, that can also be extended to nonconventional entropic formulations.
Marzari et al. (Mon,) studied this question.