[ESPResSo-users] NPT simulation

[Raffaela Cabriolu]

Dear espressoMD mailing list,

I think I have previously posted the same question in the "devel" mailing list. Sorry for that. 

I am setting 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 by Kolb and Dunweg J.Chem. Phys., 111(10):4453–59, 1999.

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:                      http://espressomd.org/html/despresso_mails.html
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

Raffaela Cabriolu, Post Doc scientist
Department of Chemistry
Norwegian University of Science and Technology (NTNU) Høgskoleringen 5,
Realfagbygget D3-126
7491 Trondheim, Norway