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[ESPResSo-devel] NPT simulations with EspressoMD

From: Raffaela Cabriolu
Subject: [ESPResSo-devel] NPT simulations with EspressoMD
Date: Mon, 6 Feb 2017 16:35:10 +0000

Dear espressoMD mailing list,

I am trying to set up a NPT simulation for colloids that are interacting via Yukawa potential and WCA potential. The manual of version (3.3.1) states that:
"for a simulation of the NPT ensemble, you need to use a standard thermostat for
the particle velocities (e.g. Langevin or DPD), and a thermostat for the box geometry (e.g. the isotropic NPT thermostat)"

I have then implemented a script with those lines:

integrate set npt_isotropic p_ext piston_mass
thermostat npt_isotropic temperature gamma0 gammaV
thermostat langevin temperature gamma

To understand how to set the friction factors and the piston mass I am referring to the paper J.Chem. Phys., 111(10):4453–59, 1999.
At this point it is not clear to me the meaning of the friction factors in the command above.
According to the paper by Kolb and Dunweg there are two damping parameters. The damping γ0 in the paper is related to the particle relaxation time while γV is related to the volume relaxation time. Are those two factors in the paper coinciding, respectively, with gamma0 and gammaV of the NPT isotropic protocol described by the command line above ?

My doubts comes from the fact that I have found a link where the value of the parameters piston_mass, gamma0 and gammaV are discussed for espressoMD NPT protocoll:            
In the link those value are given as:
setmd piston 0.0001
setmd g0 0.001
setmd gv 0.5

But g0 assumes the optimal value of γV in the paper, while 
gv assumes the optimal value of γ0 in the paper.

Furthermore, I would like to understand better the role of the "Langevin" command line. The temperature in "thermostat npt_isotropic" should be the same as in "thermostat langevin" command ? Gamma should not be equal to the the friction term for the particles velocities, then coinciding with gamma0 ?

Thank you in advance for your attention

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