Low temperature neutron diffraction and high level computational methods have been applied to investigate the short hydrogen bond in urea-phosphoric acid. It is found that isolated molecule calculations predict a 'normal' O-H . . . O hydrogen bond, in strong disagreement with the very short, 3c-4e hydrogen bond found from the neutron diffraction. Extending these calculations into a periodic environment using plane-wave DFT methods give much improved agreement with experiment, with a much shorter, stronger hydrogen bond, and significant elongation of the 0-H 'covalent' bond. (C) 2002 Elsevier Science B.V. All rights reserved.