getForce returns zero in custom ActionAtomistic subclass

[m-nader]

Hi,

I’m writing a custom action inheriting ActionAtomistic and getForce returns zero at every step. I am using Gromacs 2024.3 patched with Plumed 2.10, and what I need are the forces calculated by the MD engine. This is my code, I have tried running it both within calculate() and apply():

  Vector sumF(0.0,0.0,0.0);
  for(unsigned i=0; i<getNumberOfAtoms(); i++) {
      std::pair<std::size_t,std::size_t> a = getValueIndices( AtomNumber::index(i));
      Vector f = getForce(a);
      sumF+=f;
    }

What exact method should I use to read per-atom forces inside ActionAtomistic for PLUMED 2.10? (what argument type does getForce expect?)
Do I need to register/request forces in the constructor/initialise step? If so, what minimal code is needed?

Thanks,
Manuela