Where nuclear quantum effects can provide physical insights

As experimental techniques assessing the nuclear and electronic properties of matter become more accurate, theoretical methods also need to be updated to explain newly uncovered physical phenomena. It is becoming increasingly straightforward to use path integral methods to include nuclear quantum effects (NQE) in simulations of anharmonic systems, and so avoid the deficiencies of simple harmonic approximations.

Mariana Rossi describes the regimes where the contribution of NQE are expected to have a significant impact due to weak electronic interactions, such as hydrogen bonds and van der Waals forces. These systems call for simulation methods that adequately account for the quantum nature of electrons and nuclei.

“Because NQE are ubiquitous and because there are different new methodologies that can be employed at very affordable computational cost for most problems in chemical physics, there is no need for the community to ignore these effects in most simulations,” Rossi said.

She compiles a few rules for judging whether NQE could have an impact.

“If the answer is ‘maybe,’ then there is no computational reason not to account for them,” she said.

Notably, anharmonic contributions to nuclear motion may become very pronounced at all temperatures, especially when light atoms are involved in an interaction. The most popular methodologies to address NQE are based on the harmonic approximation, which fails for weakly bonded systems due to the large amplitude motions and distortions at play. Properly accounting for NQE can qualitatively affect a number of computational results.

Rossi believes methods that can capture the nuclear quantum dynamical properties can be improved in the near future. She looks forward to seeing these improvements applied to the understanding of hydrogen-containing systems in a variety of conditions.

Source: “Progress and challenges in Ab initio simulations of quantum nuclei in weakly bonded systems,” by Mariana Rossi, Journal of Chemical Physics (2021). The article can be accessed at https://doi.org/10.1063/5.0042572 .

This paper is part of the Special Collection in Honor of Women in Chemical Physics and Physical Chemistry, learn more here .