  The (001) surface of the emerging photovoltaic material cesium lead triiodide
(CsPbI3 ) is studied. Using first-principles methods, we investigate the atomic
and electronic structure of cubic ({\alpha}) and orthorhombic ({\gamma}) CsPbI3
. For both phases, we find that CsI-termination is more stable than
PbI2-termination. For the CsI-terminated surface, we then compute and analyse
the surface phase diagram. We observe that surfaces with added or removed units
of nonpolar CsI and PbI2 are most stable. The corresponding band structures
reveal that the {\alpha} phase exhibits surface states that derive from the
conduction band. The surface reconstructions do not introduce new states in the
band gap of CsPbI3, but for the {\alpha} phase we find additional surface
states at the conduction band edge.
