  Organic-inorganic halide perovskites present a number of challenges for
first-principles atomistic materials modelling. These `plastic crystals'
feature dynamic processes across multiple length-scales and time-scales, which
include: (i) transport of slow ions and fast electrons; (ii) highly anharmonic
lattice dynamics with short phonon lifetimes; (iii) local symmetry breaking of
the average crystallographic space group; (iv) strong relativistic (spin-orbit
coupling) effects on the electronic band structure; (v) thermodynamic
metastability and rapid chemical breakdown. These issues, which affect the
operation of solar cells, are outlined in this perspective. We also discuss
general guidelines for performing quantitative and predictive simulations of
these materials, which are relevant to metal-organic frameworks and other
hybrid semiconducting, dielectric and ferroelectric compounds.
