Vibrational force constants for diatomic molecules from rigid pseudoatoms

The quadratic vibrational force constant for a diatomic molecule can be evaluated from generalized x‐ray scattering factors if the expansion contains at least dipole scattering factors and if the first derivative of these functions with respect to R are known. Rigid pseudoatoms are defined as the finite multipole set, [L‖K], of generalized x‐ray scattering factors determined at R_e. For this case, the quadratic force constant estimate depends on the pseudoatom charge density on the neighboring nucleus and on the pseudoatom contribution to the field gradient about the neighboring nucleus as shown by Bader and Bandrauk [J. Chem. Phys. 49, 1666 (1968)]. A previous estimate for H₂ is shown to be in error. The estimates of k_e for N₂, CO, BF, and FH from [2‖2] rigid pseudoatoms are calculated and prove to be too large by more than 50%. The radial relaxation terms reduce these values to the correct k_e.