[CP2K-user] [CP2K:18479] Restraint Unit

[Yuqing Fu]

Dear all,

I'm trying to carry out umbrella sampling of a dihedral angle (torsion) 
using the MOTION / CONSTRAINT / COLLECTIVE / RESTRAINT section in CP2K. I 
came across a problem with the unit of the force constant K. I understand 
that users can specify energy units like kJ/mol to the constant, but the 
actual unit of the constant should contain the unit of the torsion angle as 
well, say kJ/(mol-rad^2) or kJ/(mol-deg^2). I am wondering what unit is 
used for the torsion part in the unit of K. Does it use the same 
user-specified unit for TARGET when running MD simulations? Or does it use 
the default angle unit for the constant?

As an example, below is my input for the CONSTRAINT section:

  &CONSTRAINT
    &COLLECTIVE
      COLVAR 1
      TARGET [deg] 10
      INTERMOLECULAR .TRUE.
      &RESTRAINT
         K [kjmol] 100.0
      &END RESTRAINT
    &END COLLECTIVE
  &END CONSTRAINT

from which my intention was to use a 100 kJ/(mol-deg^2) force constant for 
my bias. Yet what I found from my restart file (generated automatically by 
CP2K using its default unit) is:

   &CONSTRAINT
     &COLLECTIVE
       COLVAR  1
       INTERMOLECULAR  T
       TARGET     1.7453292519943295E-01
       &RESTRAINT
         K     3.8087988796022196E-02
       &END RESTRAINT
     &END COLLECTIVE
     &COLVAR_RESTART
           1.7453292519943295E-01
     &END COLVAR_RESTART
   &END CONSTRAINT

in which the force constant seems to be 100 kJ/(mol-rad^2), which is 0.03 
kJ/(mol-deg^2). 
(K = 3.808E-02 hartree = 100kJ/mol, and TARGET = 1.745E-01 rad = 10 degree)

I would really appreciate any help in understanding the units here. Thank 
you!

With regards,
Yuqing Fu

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