Dynamics of two tetradecamer DNA with AT- and GC alternating sequences, respectively, were simulated in explicit aqueous solution using earlier described protocol.For each duplex, nine 164 ns trajectories of all-atom dynamics were computed with fixed torque values in the range 20 pN nm, which gives about 3 s of simulations in total. Three additional trajectories were computed for the GC alternating fragment for verification. Below we consider only evaluation of the torsional stiffness. Other methods and protocols are described elsewhere . In the harmonic approximation the torsional free energy of a DNA fragment of length L subjected to external torque is