Two-legged approximation for building non-empirical hybrids and analyzing correlation at finite temperature

Abstract: Warm dense matter is a highly energetic phase characterized by strong correlations, thermal effects, and quantum effects of electrons. Thermal density functional theory is commonly used in simulations of this challenging phase, driving the development of temperature-dependent approximations to the exchange-correlation free energy. In this work, a finite-temperature extension of the two-legged adiabatic connection construction is demonstrated for the uniform electron gas and asymmetric Hubbard dimer at warm dense matter conditions. This provides the structure of a temperature- and density-dependent weighting scheme for a hybrid exchange-correlation approximation. The construction also provides evidence that nonlinear thermal effects on the balance between exchange and correlation, as well as that between kinetic, entropic, and potential components of the correlation, persist and can even be emphasized by strong electron-electron interaction. These findings point additionally to a complicated interplay between temperature, density, and interaction strength in the strong correlation character of these model systems.