  Black Phosphorus (BP) has gained renewed attention due to its singular
anisotropic electronic and optical properties that might be exploited for a
wide range of technological applications. In this respect, the thermal
properties are particularly important both to predict its room temperature
operation and to determine its thermoelectric potential. From this point of
view, one of the most spectacular and poorly understood phenomena is, indeed,
the BP temperature-induced band-gap opening: when temperature is increased the
fundamental band-gap increases instead of decreasing. This anomalous thermal
dependence has also been observed, recently, in its monolayer counterpart. In
this work, based on \textit{ab-initio} calculations, we present an explanation
for this long known, and yet not fully explained, effect. We show that it
arises from a combination of harmonic and lattice thermal expansion
contributions, which are, in fact, highly interwined. We clearly narrow down
the mechanisms that cause this gap opening by identifying the peculiar atomic
vibrations that drive the anomaly. The final picture we give explains both the
BP anomalous band-gap opening and the frequency increase with increasing volume
(tension effect).
