The Ca(${\mathrm{ND}}_{3}$${)}_{6}$ compound is a metallic crystal which is subject to considerable diffusional motion above 40 K; at lower temperature the protons are in a disordered state (molecular glass). It is then very difficult to refine the molecular structure of the ammonia group from classical Bragg scattering. The geometry of the ${\mathrm{ND}}_{3}$ group has been measured with high precision in Ca(${\mathrm{ND}}_{3}$${)}_{6}$ as well as in liquid and solid deuteroammonia from diffuse scattering using neutron scattering at very large momentum-transfer vector (up to 23 A${\mathrm{\r{}}}^{\mathrm{\ensuremath{-}}1}$). It is found that the tetrahedral geometry of ${\mathrm{ND}}_{3}$ in the compound is the same as in liquid ammonia; the bond length ${\mathit{R}}_{\mathrm{N}\mathrm{\ensuremath{-}}\mathrm{D}}$ in the compound is nevertheless significantly longer than in the gas phase (${\mathit{R}}_{\mathrm{N}\mathrm{\ensuremath{-}}\mathrm{D}}$\ensuremath{\approxeq}1.025 \AA{} as compared with 1.01 \AA{}) while the bond angle is decreased (D-N-D\ensuremath{\approxeq}103\ifmmode^\circ\else\textdegree\fi{} instead of 106\ifmmode^\circ\else\textdegree\fi{}). The present results are in contradiction with the model of distorted ammonia as proposed by Von Dreele et al. for this compound.