Representing the thermal state in time-dependent density functional theory
- Sandia National Laboratories, Albuquerque, New Mexico 87185-1315 (United States)
- Advanced Logic Lab, Samsung Semiconductor, Inc., Austin, Texas 78754 (United States)
Classical molecular dynamics (MD) provides a powerful and widely used approach to determining thermodynamic properties by integrating the classical equations of motion of a system of atoms. Time-Dependent Density Functional Theory (TDDFT) provides a powerful and increasingly useful approach to integrating the quantum equations of motion for a system of electrons. TDDFT efficiently captures the unitary evolution of a many-electron state by mapping the system into a fictitious non-interacting system. In analogy to MD, one could imagine obtaining the thermodynamic properties of an electronic system from a TDDFT simulation in which the electrons are excited from their ground state by a time-dependent potential and then allowed to evolve freely in time while statistical data are captured from periodic snapshots of the system. For a variety of systems (e.g., many metals), the electrons reach an effective state of internal equilibrium due to electron-electron interactions on a time scale that is short compared to electron-phonon equilibration. During the initial time-evolution of such systems following electronic excitation, electron-phonon interactions should be negligible, and therefore, TDDFT should successfully capture the internal thermalization of the electrons. However, it is unclear how TDDFT represents the resulting thermal state. In particular, the thermal state is usually represented in quantum statistical mechanics as a mixed state, while the occupations of the TDDFT wavefunctions are fixed by the initial state in TDDFT. We work to address this puzzle by (A) reformulating quantum statistical mechanics so that thermodynamic expectations can be obtained as an unweighted average over a set of many-body pure states and (B) constructing a family of non-interacting (single determinant) TDDFT states that approximate the required many-body states for the canonical ensemble.
- OSTI ID:
- 22415864
- Journal Information:
- Journal of Chemical Physics, Vol. 142, Issue 20; Other Information: (c) 2015 AIP Publishing LLC; Country of input: International Atomic Energy Agency (IAEA); ISSN 0021-9606
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ORGANIC
PHYSICAL AND ANALYTICAL CHEMISTRY
ATOMS
CAPTURE
COMPARATIVE EVALUATIONS
DENSITY FUNCTIONAL METHOD
ELECTRON-ELECTRON COUPLING
ELECTRON-PHONON COUPLING
ELECTRONS
EQUATIONS OF MOTION
EXCITATION
GROUND STATES
MANY-BODY PROBLEM
MIXED STATE
MIXED STATES
MOLECULAR DYNAMICS METHOD
PURE STATES
QUANTUM MECHANICS
THERMALIZATION
THERMODYNAMIC PROPERTIES
TIME DEPENDENCE
WAVE FUNCTIONS