Lowering of the Kinetic Energy in Interacting Quantum Systems
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Lowering of the Kinetic Energy in Interacting Quantum Systems

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B. Militzer and E.L. Pollock

Interactions never lower the ground state kinetic energy of a quantum
system below the noninteracting value. However, at nonzero
temperature, where the system occupies a thermal distribution of
states, interactions can reduce the kinetic energy. This can be
demonstrated from a first order weak coupling expansion. Simulations
(both variational and restricted path integral Monte Carlo) of the
electron gas model and dense hydrogen confirm this and show that in
contrast to the ground state case, at nonzero temperature the
population of low momentum states can be increased relative to the
free Fermi distribution. This effect is not seen in simulations of
liquid He-3.

*Phys. Rev. Lett.* **89** (2002) 280401.