Rms computes a RMS fit between two atom selections, but does not tranform the models after performing the fit.
USAGE
rms (selection), (target-selection)
EXAMPLES
fit ( mutant and name ca ), ( wildtype and name ca )
To determine the RMS without any fitting, see Rms_Cur
Fit, Rms, Rms_Cur are finicky and only work when all atom identifiers match: segi, chain, resn, name, alt. If they don't then you'll need to use the alter command to change the identifiers to that they do -- typically that means clearing out the SEGI field, renaming chains, and sometimes renumbering.
I tried made two selections A, and D as
PyMOL>sel A, 1gh2 and n. CA and i. 65-99
Selector: selection "A" defined with 35 atoms.
PyMOL>sel D, 1kao and n. CA and i. 64-98
Selector: selection "D" defined with 35 atoms
which as you can see both yield 35 atoms. Now,
rms_cur A, D
won't work, due to the aforementioned reason. To fix this, one needs ..→