Full relativistic calculations of the quadrupole and electric field gradients for C₂, N₂, and O₂

In the present work we calculate the energies, quadrupole moments, and electric field gradients (EFGs) of molecules C₂, N₂, and O₂ based on DIRRCI method with basis aug-cc-pVTZ-DK. We prove that the quadratic force constant k₂ is the product of charge and EFG at its equilibrium nuclear distance. The dipole charge distributions for these symmetrical molecules are all in equilibrium, however, the quadrupole charge distributions are far from equilibrium, among these, there is the most remarkable deviation from equilibrium for N₂, for its many charges concentrate on two sides of molecule, which is in agreement with the well-known characteristic of nitrogen molecule. The relativistic effect is remarkable even for the same period.