  Here we develop a microscopic approach aimed at the description of a suite of
physical effects related to carrier transport in, and the optical properties
of, halide perovskites. Our theory is based on the description of the nuclear
dynamics to all orders and goes beyond the common assumption of linear
electron-phonon coupling in describing the carrier dynamics and band gap
characteristics. When combined with first-principles calculations and applied
to the prototypical MAPbI$_{3}$ system, our theory explains seemingly disparate
experimental findings associated with both the charge-carrier mobility and
optical absorption properties, including their temperature dependencies. Our
findings demonstrate that orbital overlap fluctuations in the lead-halide
structure plays a significant role in determining the optoelectronic features
of halide perovskites.
