First principles atomistic theory of halide perovskites

Numerical modeling methods that do not rely on empirical input but only on the first principles of physics can provide atomistic understanding, validate experimental observations, and predict emerging properties and new materials. Halide perovskites have equally challenged and inspired researchers that develop and use such methods. Their soft and highly anharmonic lattices, featuring rotating molecules and migrating ions, intricate coupling between structural and electronic degrees of freedom, and complex electronic and excited state structures, call for computer experiments at the verge of (and often beyond) technical feasibility. This has led to the development of new models, new approaches, and a general push for expanding the applicability of state-of-the-art first-principles techniques. This chapter aims to assist newcomers to the field in understanding the basic theory of common first-principles methods, with a focus on the challenges associated with practical calculations of structural and optoelectronic properties of halide perovskites.