Calculation of the vibrational dependence of the electric field gradient in HD

We calculate the deuterium electric field gradient in the HD and D ₂ molecules at various internuclear separations by making use of a previously proposed variation - perturbation method. We calculate DEFG values at internuclear separations R = 1.2, 1.3, 1.4, 1.5, and 1.6. We also calculate the average values for the vibrational ground state v = 0 and for the vibrational states J = 0, J = 1, and J = 2 of the HD and D₂ molecules. The vibrational averaged DEFG value is very close to the equilibrium value at R = 1.4 due to accidental cancellation of different contributions. The difference between our result and other theoretical values is about 2.5%.