Quantum nuclear vibrations and the electronic properties of molecular crystals

We present a study of the electronic properties of molecular crystals, with a focus on the effect of nuclear quantum motion and anharmonicity on their band gap. 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 the electronic properties at the DFT level of theory, by coupling first principles molecular dynamics with a nuclear quantum thermostat, following a protocol previously applied to diamond and amorphous carbon [1]. We use the Qbox code (http://qboxcode.org/) coupled with i-PI (http://ipi-code.org/). 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 when using the frozen phonon (FP) approximation, which neglects inter-molecular anharmonic effects, we introduce a large error (~ 50%) in the calculation of the band gap ZPR. Instead, when using a stochastic method [2], 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 largely 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.

[1] A. Kundu et al, Phys. Rev. Mat. 2021 & PNAS 2022; [2] M. Zacharias, F. Giustino. Phys. Rev. B 2016.