Electron density distributions, derived from ab initio molecular wavefunctions, have been calculated for CN− and SCN− ions. From these dynamic densities were calculated assuming rigid body thermal vibrations of the molecules. Comparison with the difference density in NaCN - 2H2O, NaSCN and NH4SCN, observed by X-ray diffraction, is fair. Remaining differences between theory and experiment are discussed. The Hartree-Fock method gives a better correspondence with the observed electron density distribution in the thiocyanate ion than the Hartree-Fock-Slater method.