Grains involved in valent interactions. Blue lines denote valent (harmonic)
bonds, red arcs mark valent angles, bold blue lines are axes of rotation in the
torsion potentials. The circles marked as N stand for original atoms N9 on A,G
bases and N1 on T,C bases (no coarse grains are centered on these atoms, their
coordinates are calculated from the positions of the three grains that define
the base plane).