This is part of the generic module

Dump the force acting on one of a values in a file.

For a CV this command will dump the force on the CV itself. Be aware that in order to have the forces on the atoms you should multiply the output from this argument by the output from DUMPDERIVATIVES. Furthermore, also note that you can output the forces on multiple quantities simultaneously by specifying more than one argument. You can control the buffering of output using the FLUSH keyword.

Compulsory keywords
ARG the input for this action is the scalar output from one or more other actions. The particular scalars that you will use are referenced using the label of the action. If the label appears on its own then it is assumed that the Action calculates a single scalar value. The value of this scalar is thus used as the input to this new action. If * or *.* appears the scalars calculated by all the proceding actions in the input file are taken. Some actions have multi-component outputs and each component of the output has a specific label. For example a DISTANCE action labelled dist may have three componets x, y and z. To take just the x component you should use dist.x, if you wish to take all three components then use dist.*.More information on the referencing of Actions can be found in the section of the manual on the PLUMED Getting started. Scalar values can also be referenced using POSIX regular expressions as detailed in the section on Regular Expressions. To use this feature you you must compile PLUMED with the appropriate flag.
STRIDE ( default=1 ) the frequency with which the forces should be output
FILE the name of the file on which to output the forces
RESTART allows per-action setting of restart (YES/NO/AUTO)
UPDATE_FROM Only update this action from this time

Only update this action until this time

The following input instructs plumed to write a file called forces that contains the force acting on the distance between atoms 1 and 2.

(See also DISTANCE)