If OCTTargetOperator = oct_tg_velocity, then one must supply the
target to optimize in terms of the ionic velocities. This is done by
supplying a string through the block OCTVelocityTarget.
Each velocity component is supplied by "v[n_atom,vec_comp]",
where n_atom is the atom number, corresponding to the
Coordinates block, and vec_comp is the corresponding
vector component of the velocity. The target string can be
supplied by using several lines in this block.
As an example, the following target can be used to maximize the
velocity difference between atom 1 and 2 (in a 3D system):