The molecular difference density in tetracyanocyclobutane, expanded in a flexible basis of localized deformation functions centred on the atoms, has been derived by least-squares refinement from X-ray data on a crystal cooled to about 165K. The 1781 independent reflexions recorded diffractometrically with Mo K~ radiation have permitted the simultaneous determination of the charge distribution and the anisotropic atomic vibrations despite large correlations between the corresponding parameters. An attempt to derive the internal vibrations from an empirical force field was only partly successful. Bond lengths have been corrected for internal vibration, which is particularly significant for the C~---N bond. The cyclobutane ring is planar and square. The observed deformation density near the bonds closely matches theoretical maps of the corresponding portions of the molecules cyclobutane, HCN and NCCN. Sharp peaks and shallow valleys around each atomic centre conform to a consistent pattern of angular polarization that builds up a few bond and lone-pair maxima at the expense of a diffuse nearly spherical envelope of charge depletion. Gross charge displacements result in a moderately polar --4" -I---C-H bond and a strongly polar C~-N group.

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This model could simplify the Bayesian monitoring of cyclosporin therapy.