# espressopp.interaction.ConstrainCOM¶

This class is for calculating forces acting on constrained center of mass of subchains [Zhang_2014].

Subchains are defined as a tuple list.

$U = k_{com} \left(\vec{r_{com}} - \vec{R_{com}}\right)^2,$

where $$\vec{r_{com}}$$ stands for the center of mass of subchain and $$\vec{R_{com}}$$ stands for the desired center of mass of subchain.

This class implies 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.
 [Zhang_2014] G. Zhang, L. A. Moreira, T. Stuehn, K. Ch. Daoulas, and K. Kremer, Equilibration of high molecular weight polymer melts: A hierarchical strategy, Macro Lett., 2014, 3, 198
espressopp.interaction.ConstrainCOM(k_com)
Parameters: k_com (real) – (default: 100.)
espressopp.interaction.FixedLocalTupleListConstrainCOM(system, tuplelist, potential)
Parameters: system – tuplelist – potential –
espressopp.interaction.FixedLocalTupleListConstrainCOM.getPotential()
espressopp.interaction.FixedLocalTupleListConstrainCOM.setCom(particlelist)
Parameters: particlelist –