  Data analysis of the next generation effective antineutrino mass measurement
experiment KATRIN requires reliable knowledge of systematic corrections. In
particular, the width of the daughter molecular ion excitation spectrum
rovibrational band should be known with a better then 1% precision. Very
precise ab initio quantum calculations exist, and we compare them with the well
known tritium molecule parameters within the framework of a phenomenological
model. The rovibrational band width with accuracy of a few percent is
interpreted as a result of the zero-point atomic oscillation in the harmonic
potential. The Morse interatomic potential is used to investigate the impact of
anharmonic atomic oscillations. The calculated corrections cannot account for
the difference between the ab initio quantum calculations and the
phenomenological model.
