  We inspect the origin of the inverted singlet-triplet gap (INVEST) and slow
change in the reverse intersystem crossing (rISC) rate with temperature, as
recently observed. A Wigner phase space study reveals, that though INVEST is
found at equilibrium geometry, variation in the exchange interaction and the
doubles-excitation for other geometries in the harmonic region leads to non-
INVEST behavior. This highlights the importance of nuclear degrees of freedom
for the INVEST phenomenon and in this case, geometric puckering of the studied
molecule determines INVEST and the associated rISC dynamics.
