  Despite their puzzling vibrational characteristics that include strong
signatures of anharmonicity and thermal disorder already around room
temperature, halide perovskites exhibit favorable optoelectronic properties for
applications in photovoltaics and beyond. Whether these vibrational properties
are advantageous or detrimental to their optoelectronic properties remains,
however, an important open question. Here, this issue is addressed by
investigation of the {finite-temperature optoelectronic properties} in the
prototypical cubic CsPbBr$_3$, using first-principles molecular dynamics based
on density-functional theory. It is shown that the dynamic flexibility
associated with halide perovskites enables the so-called transversality, which
manifests as a preference for large halide displacements perpendicular to the
Pb-Br-Pb bonding axis. We find that transversality is concurrent with
vibrational anharmonicity and leads to a rapid rise in the joint density of
states, which is favorable for photovoltaics since this implies sharp optical
absorption profiles. These findings are contrasted to the case of PbTe, a
material that shares several key properties with CsPbBr$_3$ but cannot exhibit
any transversality and, hence, is found to exhibit much wider band-edge
distributions. We conclude that the dynamic structural flexibility in halide
perovskites and their unusual vibrational characteristics might not just be a
mere coincidence, but play active roles in establishing their favorable
optoelectronic properties.
