The influence of intermolecular interactions on the electron-density distribution. A comparison of experimental and theoretical results for a-oxalic acid dihydrate

The experimental electron-density distribution in a~toxalic acid dihydrate, a-C2H204.2H20, as measured by Dam, Harkema & Feil [Acta Cryst. (1983), B39, 760-768], is compared with results from a theoretical density-functional calculation, with a local approximation to exchange and correlation. The agreement between the multipole-refined experimental and the refined vibrationally averaged theoretical electrondensity distribution improves significantly when taking into account the effects of hydrogen bonding and crystal environment. A comparison of structure factors based on the experimental electron-density distribution with those based on the vibrationally averaged theoretical molecular-density distributions, yielded an R factor of 1.3%. Inclusion of the effects of hydrogen bonding in the theoretical model lowered the R factor to 1.1%. When the effects of the crystal environment were taken into account, a further lowering to 1.0% resulted.