Hybrid density functional theory calculations are used to investigate the strength of hydrogen bonds of structurally different molecules in complex with a standard donor and acceptor in vacuo. B3LYP/aug-cc-pVDZ calculations with one angle constraint lead to excellent correlations with experimental data (R(2) = 0.94, s(y) = 0.45 for acceptors and R(2) = 0.77, s(y) = 0.88 for donors). Substitutions of aromatic systems by electron donating and -withdrawing groups show a reinforcement of the interaction when substituting an acceptor with electron donating groups and weakening by substitution with electron withdrawing groups. For donor systems the opposite effect can be observed. Drug design of novel ligands will be able to profit from the predictive power of the method established, as hydrogen bonds between receptor and drug molecules are an important criterion for binding affinities.