# espressopp.interaction.ConstrainRG¶

This class calculates forces acting on constrained radii of gyration of subchains [Zhang_2014].

Subchains are defined as a tuple list.

$U = k_{rg} \left(R_{g}^2 - {R_{g}^{ideal}}^2\right)^2$

where $$R_{g}^{ideal}$$ stands for the desired radius of gyration of subchain.

This class set 2 conditions on a tuple list. defining subchains.

1. The length of all tuples must be the same.
2. int(key particle id / The length of a tuple) must not be redundantly, where key particle id is the smallest particle id in a tuple.
espressopp.interaction.ConstrainRG(k_rg)
Parameters: k_rg (real) – (default: 100.)
espressopp.interaction.FixedLocalTupleListConstrainRG(system, tuplelist, potential)
Parameters: system – tuplelist – potential –
espressopp.interaction.FixedLocalTupleListConstrainRG.getPotential()
espressopp.interaction.FixedLocalTupleListConstrainRG.setRG(particlelist)
Parameters: particlelist (python::list) –