The velocity-rescaling option (IEQFrq not zero) periodically rescales the velocities. It uses the equations
where
n
is the number of free atoms in the system, 
is Boltzmann's constant,
is the temperature in K specified by
the FINAltemperature parameter, 
denotes
the velocity of atom i at
time t, and angle brackets enclose a trajectory average over the time intervals
between the rescalings.