Ortho-para transition in molecular hydrogen

The radiative ortho-para transition in molecular hydrogen is studied. This highly forbidden transition is very sensitive to relativistic and subtle nonadiabatic effects. Our result for the transition rate in the ground vibrational level Gamma(J=1 --> J=0) = 6.20(62) x 10(-14) yr(-1) is significantly lower in comparison to all the previous approximate calculations. Experimental detection of such a weak line by observation of, for example, cold interstellar molecular hydrogen is at present unlikely.