DHA, a program for calculating hydrogen atom coordinates of solvent atom involved in hydrogen bonding interactions

Generally it is difficult to locate hydrogen atoms accurately using X-ray data because of their low scattering power and lack of core electrons. For many purposes the hydrogen positions can be calculated according to well-established geometrical criteria. Nevertheless, in some cases the positioning of the hydrogen atoms is tricky. Usually the problem arises when solvent molecules (e.g. water, OH groups or ammonia) are present in the crystal structure. Fortunately, hydrogen bonding interaction are characterized by an appropriate geometry and distance between the donor (D) and acceptor (A) atoms and requires the participation/presence of a hydrogen atom (D - H center dot center dot center dot A). Thus, if the coordinates of the donor and acceptor atoms are known one can calculate the coordinates of the "elusive" hydrogen atom. The program DHA uses an extremely simple mathematical apparatus to produce the coordinates of the hydrogen atom (H) participating in a D - H center dot center dot center dot A type of interaction.