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Ab Initio Quantum Monte Carlo Simulation of Warm Dense Electrons

Tobias Dornheim, Christian-Albrechts-Universität zu Kiel

18.10.2018 at 10:00 

Over the last years, there has emerged an increasing interest in warm dense matter, an
exotic state with high densities (n~10 21 – 10 27 cm -3 ) and temperatures (T~10 4 –10 8 K) that
occurs in astrophysical objects (e.g., giant planet interiors) and is relevant for cutting edge
technologies like inertial confinement fusion.
Despite the remarkable experimental progress at large research facilities, a thorough
theoretical description of matter under these extreme conditions is notoriously difficult due
to the intricate interplay of 1) Coulomb coupling, 2) thermal excitation, and 3) quantum
degeneracy effects. Because of the fermionic nature of the electrons, ab initio path integral
Monte Carlo (PIMC) methods, which are in principle capable to provide an exact description,
are severely hampered by the fermion sign problem.
In this talk, I present a new strategy to simulate warm dense electrons over a broad
parameter range by combining two different PIMC techniques that are efficient at
complementary parameter ranges. In combination with a new extrapolation procedure to
reach the thermodynamic limit from a finite simulation box, this has allowed us to construct
an accurate exchange-correlation functionial for electrons in the warm dense regime that
can directly be used in, e.g., density functionial theory simulations.
A related question is the response of the electrons to an external perturbation, which can
often be treated accurately in the framework of linear response theory. To address this
question, I discuss the possibility to directly simulate a perturbed inhomogenous system
and the utility of imaginary-time correlation functions. The latter are used as input for an
analytic continuation to compute the first ab initio results for the dynamic structure
factor---a key quantity in the diagnostics of warm dense matter experiments.

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