  We succeeded in remarkably nice theoretical description of the temperature
dependence of the paramagnetic susceptibility of Ba$_2$YMoO$_6$ in the whole
temperature range as originating from the atomic-scale discrete electronic
structure of Mo$^{5+}$ ions in the 4d$^1$ configuration resulting from
crystal-field and spin-orbit interactions. A strong violation of the
Curie-Weiss law and a nonmagnetic state of BYMo experimentally-observed down to
50 mK is caused by the dramatical reduction of the Mo-ion magnetic moment. The
quantitative reproduction of $\chi$(T) proves that almost all Mo ions are in
the Mo$^{5+}$ state over the whole temperature range.
