The nitric acid-water complex is studied by high level ab initio calculations. The equilibrium structure of the complex has an asymmetric doubly hydrogen-bonded ring. A strong hydrogen bond is donated by the hydroxyl group of nitric acid to the oxygen atom of water, and a much weaker hydrogen bond is donated by an OH bond of water to a second oxygen of nitric acid. The HNO3 unit in the complex is considerably distorted with the OH stretching frequency red-shifted by over 300 cm-1 from the isolated molecule and the infrared intensity enhanced by an order of magnitude. The binding energy of the complex is calculated as De=-9.5 kcal/mol. The equilibrium fraction of nitric acid complexed with water is predicted to be near 1% at the Earth's surface and falls with the increase of altitude. ¿ American Geophysical Union 1996