This option is turned on by TCOUpling. This method was described by Berendsen et al. (1984). It is essentially a Langevin-type method with zero random forces and a scaled friction coefficient. The friction coefficient is computed by the program from the equation
where 
is given by the FBETa atom property.
Note that FBETa is by default zero; i.e., FBETa has to be
specified in order to use TCOUpling.
The target temperature 
is specified by the TBATh parameter.
is the
temperature of the system at
the particular step where 
is the number of degrees of freedom of
the system. Thus, a positive fricition coefficient is applied
if the system is too hot, a negative friction coefficient is
applied if the system is too cold, and no friction is applied if
T matches 
.