cmd.intra_fit( string selection, int state )
Warren provided a great example on the PyMOL list. If the NMR ensemble has all the structures loaded as multiple states (which is the default behavoir (see split_states) for multimodel PDBs) then we can simply call:
print cmd.intra_fit(selection, state)
which will fit all of the other states to the indicated states and return the RMS values as a list.
A simple example is:
fetch 1i8e, async=0
print cmd.intra_fit("1i8e////CA", 1)