The free energy of association of adenine-thymine and guanine-cytosine base pairs is estimated from potential of mean force computations in water using the AMBER potential energy function. The results, which lead to free energies per hydrogen bond of roughly 2 kcal/mol, highlight the existence of secondary water-bridged minima for both base pairs and also show that adenine-thymine pairs undergo frequent exchanges between canonical and reversed Watson-Crick states by contra-rotation of the bases around their common axis.