Hi everyone,

I've got a question about the unit of pressure in charmm. I did a simulation of a box of water molecules in NPT ensemble by using 'cpt' command lines as below:

pcons pmass 360 pgamma 20 pref 1.0 - ! since there are 1000 water molecules
! in the system, I set pmass as 360

I am sure the 'pref' value uses the unit of 'atm'. But when I read my output file to check if the simulation result was reasonable, I was just confused by the unit it used.

In the output file, the system pressure is recorded like this:

AVER PRESS: VIRE VIRI PRESSE PRESSI VOLUme
LAVE PRESS> 478.01444 -1155.66231 -1112.90548 23.11227 29451.39517

The value of inert pressure 23.11227 is definitely not in the unit of atm. I tried to convert it into an AKMA unit, but it is still not reasonable.

1 atm = 101325 N/m^2
1 N = 1.4388 kcal/mol-A
1 m^2 = 1E20 A^2

1 atm = 101325*1.4388*1E-20 kcal/mol-A^3 =1.4579E-15 kcal/mol-A^3

Are there any mistakes in my unit conversion? Or just because in the output file it is a totally different unit for pressure?
If it really uses atm for pressure in output file, then what was wrong with my simulation?

I thought it would be a very basic question, but I just couldn't find any relevant topic after searching through the forum.

Thanks for any suggestions!

Giselle