Hi,

In plumed 1 the units are adapted to the default units of the MD code you are using (in your case GROMACS uses nm for the distance, rad for the angles, and KJ/mol for the energy).

Usually with setting KAPPA value you define some barriers (Walls or an external potential) in which your collective variable (cv) is restricted.

The functional form of the wall applied is:

V(cv) = KAPPA ((cv - LIMIT + OFF)/EPS)**EXP

In this way the restraining potential starts to act when your cv overcome the limit of your wall.

So, if you want to limit the movements of your cv below 15 A using an external potential of 100 KJ/mol:

UWALL CV 1 LIMIT 1.5 KAPPA 100.0

In this string OFF, EXP and EPS are taken as default values: EXP=4, OFF=0, EPS=1.

Hope this helps

Carmine