I am trying to add an additional harmonic restraint to a system to calculate the free energy of rotation. I am using the following approach:
( where kx and x0 are set to numerical values previously )
torsion = CustomTorsionForce('kx*(x - x0)^2')
torsion.addPerTorsionParameter('x0')
torsion.addPerTorsionParameter('kx')
system.addForce(torsion)
torsion.addTorsion(1106,1108,1111,1112,[])
However, when I try to setup a simulation using the following:
simulation = Simulation(pdb.topology,system,integrator,platform,properties)
I receive an error message:
Traceback (most recent call last):
File "smd.py", line 42, in <module>
simulation = Simulation(pdb.topology,system,integrator,platform,properties)
File "/home/g/gmgray2/.local/lib/python2.7/site-packages/simtk/openmm/app/simulation.py", line 79, in __init__
self.context = mm.Context(system, integrator, platform, platformProperties)
File "/home/g/gmgray2/.local/lib/python2.7/site-packages/simtk/openmm/openmm.py", line 4870, in __init__
this = _openmm.new_Context(*args)
Exception: CustomTorsionForce: Wrong number of parameters for torsion 0
[504313 refs]
How can I fix this? Thanks in advance
Geoffrey