<ForceField>
 <Script>
import openmm
import openmm.app as app
import openmm.unit as unit

# Find the NonbondedForce.  We need it to look up charges, and also to set the reaction field dielectric to 1.

nonbonded = [f for f in sys.getForces() if isinstance(f, openmm.NonbondedForce)]
if len(nonbonded) != 1:
    raise ValueError('Implicit solvent requires the System to contain a single NonbondedForce')
nonbonded = nonbonded[0]
nonbonded.setReactionFieldDielectric(1)

# Construct the CustomGBForce.

from openmm.app.internal.customgbforces import GBSAGBnForce
argMap = {'soluteDielectric':'soluteDielectric', 'solventDielectric':'solventDielectric', 'implicitSolventKappa':'kappa'}
solventArgs = {'SA':'ACE'}
for key in argMap:
    if key in args:
        solventArgs[argMap[key]] = args[key]
if nonbondedMethod != app.NoCutoff:
    solventArgs['cutoff'] = nonbondedCutoff.value_in_unit(unit.nanometers)
force = GBSAGBnForce(**solventArgs)
params = GBSAGBnForce.getStandardParameters(topology)
for i, p in enumerate(params):
    charge, sigma, epsilon = nonbonded.getParticleParameters(i)
    force.addParticle([charge, p[0], p[1]])
force.finalize()

# Set the nonbonded method and cutoff distance.

if nonbondedMethod == app.NoCutoff:
    force.setNonbondedMethod(openmm.CustomNonbondedForce.NoCutoff)
elif nonbondedMethod == app.CutoffNonPeriodic:
    force.setNonbondedMethod(openmm.CustomNonbondedForce.CutoffNonPeriodic)
    force.setCutoffDistance(nonbondedCutoff)
elif nonbondedMethod == app.CutoffPeriodic:
    force.setNonbondedMethod(openmm.CustomNonbondedForce.CutoffPeriodic)
    force.setCutoffDistance(nonbondedCutoff)
else:
    raise ValueError("Illegal nonbonded method for use with implicit solvent")
sys.addForce(force)
 </Script>
</ForceField>