  The stability of four fermionic particles with unit charge, of which, two are
positively, and two negatively charged, is discussed. Except for using the
simplest approximation of a single Gaussian orbital per particle, the problem
is exactly solved variationally and, by varying the masses to simulate
molecular di-hydrogen, mono-muonated di-hydrogen and di-muonated di-hydrogen,
employed to illustrate the celebrated Born-Oppenheimer approximation on the
occasion of its 90th anniversary. It is suggested that it is valid only for
di-hydrogen.
