  We study the light-absorbing states of the mixed-halide perovskite
CH$_{3}$NH$_{3}$PbI$_2$Cl and tri-iodide perovskite CH$_{3}$NH$_{3}$PbI$_3$
with density functional and many-body calculations to explain the desirable
photovolatic features of these materials. The short-lived electron-hole bound
states produced in this photovoltaic material are of halide to lead electron
transfer character, with a Wannier-type exciton. Bethe-Salpeter (GW+BSE)
calculations of the absorption cross section reveal strong screening of the
electron-hole interaction. The atomic character of the exciton retains
ligand-to-metal character within the visible spectrum, with differing degrees
of localization outside the unit cell. The average electron-hole separation in
the lowest exciton is found to be about 5$A^{\circ}$, slightly larger than the
Pb-I bond length. Finally, we determine the role of methylammonium's dipole in
the ultrafast relaxation by preparing an atomistic model of the picosecond
electronic dynamics in the tri-iodide, PbI$_3$. Our model allows us to identify
phonon modes which couple strongly to the electronic excitations, and explain
the picosecond timescale intra-band relaxation dynamics seen in recent
transient absorption experiments. We largely substantiate the conjectured three
band model for the dynamics, but also identify other possible relaxation
channels in the tri-iodide.
