Van der Waals Interactions and Anharmonicity in the Lattice Vibrations, Dielectric Constants, Effective Charges, and Infrared Spectra of the Organic−Inorganic Halide Perovskite CH3NH3PbI3

Using first-principles calculations, we perform a comprehensive and systematic analysis to establish the role of van der Waals (vdW) interactions and anharmonicity in the vibrational properties of the low-temperature orthorhombic phase of the hybrid perovskite CH3NH3PbI3. To this end, we consider the most common approaches for including vdW effects in our phonon calculations: the semiempirical Grimme approximations, the Tkatchenko-Scheffler dispersion corrections, and the vdW density-functional method. The vibrational normal modes are first calculated within the harmonic approximation. We consider the LDA and GGA approximations to the exchange-correlation functional and include spin−orbit coupling (SOC) effects. On top of the harmonic calculations, we also evaluate the anharmonicity of the normal modes and the phonon−phonon coupling by solving one-dimensional and two-dimensional nuclear Schrödinger equations, respectively, via the finite-displacement method. We observe that both the LDA and GGA approximations work remarkably well in describing the vibrational properties of CH3NH3PbI3. We find that vdW effects and relativistic effects do not have any significant impact on the vibrational properties of CH3NH3PbI3. Our study also reveals that the spinning modes of the organic CH3NH3+ cations carry considerable anharmonicity but that the anharmonic coupling between different modes is relatively small