The many detailed, though static, structures of protein-nucleic acid complexes have revealed the underlying structural determinants of the binding process: a high surface complementarity and a precise orientation of interacting roups. However, another important question not yet fully answered is why the molecules interact with each other. To answer such a question means that we have to rationalize a structure energetically. This requires a deconvolution of the relatively large negative value of the free energy of association (ΔG ) into its enthalpic (ΔH ) and entropic (ΔS ) contributions. Only then can one appreciate the true nature of the forces that drive the interaction of protein and nucleic acid. Despite some promising theoretical developments and the steady accumulation of experimental data, the deconvolution of ΔG into its components remains a difficult task