The fluctuation-dissipation theorem as a diagnosis and cure for zero-point energy leakage in quantum thermal bath simulations

Etienne Mangaud (1) , Simon Huppert (1) , Thomas Plé (1) , Philippe Depondt (1) , Sara Bonella (2) , Fabio Finocchi (1)

1 INSP - Institut des Nanosciences de Paris
2 EPFL - Ecole Polytechnique Fédérale de Lausanne

Quantum thermal bath (QTB) simulations reproduce statistical nuclear quantum effects via a Langevin equation with a colored random force. Although this approach has proven efficient for a variety of chemical and condensed-matter problems, the QTB, as many other semiclassical methods, suffers from zero-point energy leakage (ZPEL). The absence of a reliable criterion to quantify the ZPEL without resorting to demanding comparisons with path integral-based calculations has so far hindered the use of the QTB for the simulation of real systems. In this work, we establish a quantitative connection between ZPEL in the QTB framework and deviations from the quantum fluctuation-dissipation theorem (FDT) that can be monitored along the simulation. This provides a rigorous general criterion to detect and quantify the ZPEL without any a priori knowledge of the system under study. We then use this criterion to build an adaptive QTB method that strictly enforces the quantum FDT at all frequencies via an on-the-fly, spectrally resolved fine-tuning of the system-bath coupling coefficients. The validity of the adaptive approach is first demonstrated on a simple two-oscillator model. It is then applied to two more realistic problems: the description of the vibrational properties of a model aluminum crystal at low temperature and the simulation of the liquid-solid phase transition in a 13-atom neon cluster. In both systems, the standard QTB results are strongly altered by the ZPEL, which can be essentially eliminated using the adaptive approach.

Domaines: Chimie théorique et/ou physique

Etienne Mangaud, Simon Huppert, Thomas Plé, Philippe Depondt, Sara Bonella, et al.. The fluctuation-dissipation theorem as a diagnosis and cure for zero-point energy leakage in quantum thermal bath simulations. Journal of Chemical Theory and Computation, 2019, 15 (5), pp.2863-2880. ⟨10.1021/acs.jctc.8b01164⟩. ⟨hal-02171352⟩