## Finite Temperature Quantum Chemistry

** Felix Hummel
**

Institute for Theoretical Physics, Vienna University of Technology, Vienna, Austria

Mercredi 31 Octobre 2018, 11h00

bibliothèque LCT, tour 12 - 13, 4^{ème} étage

At zero temperature, coupled cluster theory is widely used to predict total energies, ground state expectation values and even excited states for molecules and extended systems. However, for systems with a small band gap, such as metals, the zero-temperature approximation not necessarily holds. Thermal effects may even give rise to interesting chemistry on metal surfaces. Most approaches to temperature dependent electronic properties employ finite temperature perturbation theory in the Matsubara frequency formulation. Computations require a large number of Matsubara frequencies to yield sufficiently accurate results, especially at low temperatures.

This work, and independently the work of White and Chan, proposes a coupled cluster implementation directly in the imaginary time domain on the compact interval [0,β], closely related to the thermal cluster cumulant approach of Mukherjee and coworkers. Here, the arising imaginary time dependent coupled cluster amplitude integral equations are solved in the linearized direct ring doubles approximation, also referred to as Tamm-Dancoff approximation with second order (linearized) screened exchange. In this framework, the transition from finite to zero temperature is uniform and comes at no extra costs, allowing to go to temperatures as low as room temperature.