[gmx-users] Replica exchange: exchanging pressures

[Ramon Crehuet Simon]

Dear all, 
I'd like to run a replica exchange, but instead of exchanging temperatures, I would like to exchange pressures. I know this does not help enhance sampling, but I would like to create several ensembles at different pressures and I want them to mix, as RE does. 
After preparing and equilibrating the system at each pressure, when I try to run RE, Gromacs complains that the systems are identical: 

Fatal error: 
The properties of the 8 systems are all the same, there is nothing to exchange 
For more information and tips for troubleshooting, please check the GROMACS 
website at http://www.gromacs.org/Documentation/Errors 

They are not, ref-p and compressibility are different in each md.mdp. Is there a way to tell Gromacs that it should exchange pressures? Or is this simply not implemented? 
Thanks in advance, 
Ramon