“intra_fit” fits all states of an object to an atom selection in the specified state. It returns the rms values to python as an array.

intra_fit selection [, state]

selection = string: atoms to fit

state = integer: target state

intra_fit ( name ca ) 

ON EXAMPLE 

from pymol import cmd 
rms = cmd.intra_fit("(name ca)",1) 

fit | rms | rms_cur | intra_rms | intra_rms_cur | pair_fit | cmd.intra_fit | commands