[gmx-users] off-diagonal of the tensor for pressure coupling should be zero??

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Dear Gromacs people

I am trying to run NPT simulation on a bilayer and want to know right setup 
to use anisotropic pressure coupling. 

I have seen two different cases of reference pressure.
Some people use 1 atm for off-diagonals (xy, yz, zx directions) but other 
use 0 atm.
>From Erik's answer, I could see that if the system is not allowed to deform 

to triclinic from rectangular shape, then the last three reference 
pressures should be zero.

Before I saw his answer, I already ran MD simulation with 1 atm for the 
off-diagonal of the pressure tensor. I have not seen any particular problem 
with this setup. 

Can anyone tell me what problem might have been caused becuase of the 
pressuring coupling to the off-diagonal directions? 

Thank you
Sunjoo