Quantum vibronic effects on the electronic properties of molecular crystals

Abstract

We present a study of molecular crystals, focused on the effect of nuclear quantum motion and anharmonicity on their electronic properties. We consider a system composed of relatively rigid molecules, a diamondoid crystal, and one composed of floppier molecules, NAI-DMAC, a thermally activated delayed fluorescence compound. We compute fundamental electronic gaps at the DFT level of theory, with the PBE and SCAN functionals, by coupling first-principles molecular dynamics with a nuclear quantum thermostat. We find a sizable zero-point-renormalization (ZPR) of the band gaps, which is much larger in the case of diamondoids (~ 0.6 eV) than for NAI-DMAC (~ 0.22 eV). We show that the frozen phonon (FP) approximation, which neglects inter-molecular anharmonic effects, leads to a large error (~ 50%) in the calculation of the band gap ZPR. Instead, when using a stochastic method, we obtain results in good agreement with those of our quantum simulations for the diamondoid crystal. However, the agreement is worse for NAI-DMAC where intra-molecular anharmonicities contribute to the ZPR. Our results highlight the importance of accurately including nuclear and anharmonic quantum effects to predict the electronic properties of molecular crystals.

arXiv comment54 pages, 20 figures

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Cited by in corpus (1)

• Quantum Vibronic Effects on the Excitation Energies of the Nitrogen-Vacancy Center in Diamond