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

Yuqing Fu joannaf0421 at gmail.com
Tue Feb 21 22:20:15 UTC 2023

```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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