How to use variable from LAMMPS as CV for Plumed?

[WJiangH]

Dear Plumed developer,

I am wondering what is the best way to pass the variables computed in LAMMPS to serve as a CV in PLUMED.

For example, I computed the variable called "op_1" using structure info (e.g., atoms) inside the lammps.in, and would like to use such "op_1" as an CV in the plumed.dat. Here are two ways I can image after read the plumed documents

1. modify the "fix_plumed.cpp" file in LAMMPS to include "op_1" and re-compile LAMMPS.
2. instead of calculate "op_1" in LAMMPS, I may pass structure info (e.g., atoms, box, etc) to plumed, and create a new .cpp file to calculate "op_1" to serve as the CV.

or do you have any better ideas?

Appreciate it for any suggestions.

Additionally, I noticed that in the fix_plumed.cpp in LAMMPS, while it pull in variables from MD engine, for example, p->cmd("setNatoms",&natoms); I cannot directly query the "natoms" in plumed. Did I miss something there, thank you so much!!

Best,
JJ