Ultrafast nonadiabatic relaxation of photoexcited C₆₀: Optimizing computational accuracy vis-à-vis expense

Relaxation dynamics of photoexcited electrons in fullerene materials underlies applications in fields, namely, organic photovoltaics and medical photothermal therapy. In this work, we use a computational approach of electron-phonon coupled nonadiabatic molecular dynamics (NAMD) based on density functional theory [1,2] to simulate such relaxation process in C₆₀ molecule. The methodology relies on a combination of the fewest-switches surface hopping approach and Kohn−Sham single-particle description. We calculate the femtosecond evolution of the population of initial excited states and of various intermediate states up to the band edge. Effects of excited state energy gaps and nonadiabatic couplings are studied. Two NAMD workflow tracks are adopted using, (i) QMflows-namd module [3,4] and (ii) Libra module [5], and the results are compared. The primary focus, as part of our ongoing campaign, has been to optimize the workflow to produce acceptably high accuracy results using less expensive exchange-correlation functionals.

[1] M.E. Madjet et al, Phys. Rev. Lett. 126, 183002, (2021).

[2] M. Madjet et al. J. Phys. Chem. Lett 8, 18 (2017).

[3] F. Zapata et al, J. Chem. Inf. Model. 59, 3191 (2019).

[4] E. Ali et al; https://arxiv.org/abs/2306.16386.

[5] M. Shakiba et al, Software Impacts 14 100445 (2022).