Quadrupole moment of the H2 molecule

We present an improved adiabatic calculation of the quadrupole moment of the H2 molecule and of its matrix elements between various vibrational rotational states. Particular attention is paid to making a reasonable estimate of the accuracy of transition elements between widely different vibrational states. An order of magnitude estimate of the nonadiabatic contribution to the quadrupole moment is also given and we show that it is typically of the order of a tenth of a percent of the adiabatic value