With the help of Hartree—Fock—Slater calculations in which very large basis sets are employed, the polarisation of the water molecule by an electric field is explored. The various features in the electron density distribution are encountered again in the long hydrogen bond in the water dimer, showing that polarisation is the main effect. In short hydrogen bonds, exchange repulsion is shown to be equally important. The quality of the computational method is tested by comparing the results of the calculation of the electron density distribution in the crystal of α-oxalic acid dihydrate with the results of accurate X-ray diffraction measurements. By using models in which subsequently covalent bonding, hydrogen bonding and the electrostatic crystalline field are included, the effects of the various components are explored. Only the full theoretical model gives excellent agreement with the experiment, showing the quality of the model and the sensitivity of the experiment.