Metal halide perovskites (MHPs) have garnered great attention in recent years in the field of photovoltaic solar cells due to their excellent optoelectronic properties.[1] Despite the popularity of these materials, many aspects of their physico-chemical properties are still unknown. The behaviour of simple MHPs has been studied widely using Density Functional Theory (DFT) calculations[2], but the high computational cost of DFT methods limits the study to small systems short time-scales. On the contrary, classical Molecular Dynamics (CMD) simulations overcome these limitations and can handle larger systems in much longer time scales than DFT. This approach would appear to be an excellent option to study the migration of ions and vacancy defects in MHPs[3]. However, CMD simulations suffer from other kind of limitations which make them unsuitable for studying chemical reactions or electronic properties of MHPs. An intermediate approach between DFT and CMD methods is often desirable to solve this challenge. In this regard, semi-empirical Quantum Mechanics methods, such as Density Functional Tight Binding (DFTB) combine the functionalities of describing both electrons and ions. In either case, the accuracy of the predictions of CMD and DFTB simulations depends on the ability of the potential parameters to describe the behaviour of real systems. Consequently, a parameterization of the potential energy to model the atomic interactions is needed.

In this work, we investigate the performance of CMD and DFTB simulations to obtain structural and dynamic properties of pure CsPbX₃ perovskite (with X = I and Br) and mixed MHPs containing the two halide anions. The CMD simulations use Buckingham and electrostatic potentials to model the interactions between all the ion pairs. The DFTB calculations are based on the recently proposed GFN-xTB method[4] designed to describe accurately geometries, vibrational frequencies, and noncovalent interactions. We choose this method because, due to its reasonable accuracy together with the limited number of parameters, it can straightforwardly be adjusted for a given application.

As first step, we developed a set of potential parameters capable to describe the properties of pure MHPs. To do so, we fit the parameters to reproduce the energy change induced by small deformations of the primitive cell of the cubic structures (Fig. 1 Left). We used DFT calculations as reference data and validated the obtained transferable set of potential parameters with available experimental data for pure and mixed MHPs[5] (Fig. 1 Right). At this step, the structural properties of pure and MHPs are well described with CMD and DFTB methods. We then used CMD simulations to obtain ion transport properties and DFTB-based simulations to obtain the formation enthalpies of the pure and mixed MHPs. The developed methodology is also useful to ascertain the impact of vacancy defects on the performance of MHPs for solar cell applications.