PDB2CONSTANT
This is part of the generic module

Create a constant value from a PDB input file

Examples
Glossary of keywords and components
Compulsory keywords
REFERENCE a file in pdb format containing the reference structure
NUMBER ( default=0 ) if there are multiple structures in the pdb file you can specify that you want the RMSD from a specific structure by specifying its place in the file here. If NUMBER=0 then the RMSD from all structures are computed
Options
NOARGS

( default=off ) the arguments that are being read from the PDB file are not in the plumed input

ARG read this single argument from the input rather than the atomic structure