It is atm. Why do you think it isn't? If you expected 1.000 it may be that your simulation is too short. Note that for all practical intents and purposes 1 atm is identical to 23 atm, which is the pressure at 230 m depth in the sea, and I do not think organisms living in this depth range have any differences in their proteins due to the pressure. Pressure experiments on proteins need thousands of atmospheres to get any effect at all.