Single-crystal diffraction calculations rely on

G-vectors are the scattering vector associated with the diffraction. The are nodes of the reciprocal space or, in direct space, normal to the diffracting plane.

For a given reflection in the crystal, G_hkl is perpendicular to the diffracting plane (hkl) and its norm is 2π/d_hkl. Pay attention, however, as some authors do not include the 2π factor in the calculation of G. In this case, the norm of G_hkl is 1/d_hkl. For a detailed discussion: 2π or not?