The structure and various properties of HCOO-.HOOCH (hydrogen diformate) anionic hydrogen-bonded system are studied. Single, double as well as triple hydrogen-bonded structures are obtained by geometry optimization up to the level of MP2/6-311++G(d,p) and by various density functional approaches. Optimized structures and calculated complexation energies are compared with experimental observation. At least 78% covalent character is found in the strong O-H-O-type hydrogen-bend. Multiple H-bonded structures provide calculated binding energies which are in the closest agreement with experiment. The experimental complexation energy is reproduced within almost chemical accuracy in the range of 35-37 kcal/mol, while the experimental observation is 36.8 kcal/mol. The secondary interaction like C-H.O contacts represent at least 3-4 kcal/mol energy lowering. The O-O bond distance is found in the range of 2.41-2.45 Angstrom by various density functional methods, while the experimental value is 2.45 Angstrom. The estimated counterpoise correction to basis set superposition error is also presented in the binding energies. The performance of density functional methods based on Gaussian and Slater-type basis is compared using the G92-DFT and ADF codes, respectively. For comparison the energetics of hydrogen diacetate is given as well.