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[ESPResSo-users] Temperature units in Langevin thermostat

From: Grigory Zarubin
Subject: [ESPResSo-users] Temperature units in Langevin thermostat
Date: Wed, 10 Sep 2014 15:20:41 +0200

The Espresso Tutorial says that one of the most frequent problems for newbies is the units that are used. This turned out to be the case for me too.
My aim is to simulate monovalent two component ionic liquid. Each particle is represented by a hard sphere with a charge + or - 1
So previously i had the MC simulation program written in Cython but i have doubts about the results it gives me for the structure factor, so i decided to check it using Espresso (it is especially reasonable since Espresso uses P3M and my own program has only Ewald sum).
So coming back to units. Simulation takes place in a cubic box with L = 10 and PBC in all directions. There are two parameters in my MC simulation:
1). Length - the diameter of a hard sphere R = 1
2). Dimensionless interaction strength U = e^2/(4 pi eps_0) 1/(k_b T) 1/R = l_B/R,
where e - elementary charge, eps_0 - vacuum permitivity, k_b - Boltzmann const, T - temperature and l_B - Bjerrum length.

In my simulation i have U = 1 and i want to simulate the same thing in Espresso using Langevin thermostat, WCA potential to mimic the hard core and P3M Coulomb for electrostatic interaction.
So my question is what should i choose for parameters of Langevin and P3M to have the same situation as in MC simulation?
As far as i understood, Langevin parameter "temperature" equals actually (k_B T), is it correct?
If yes, than i have to choose Bjerrum length in P3M which equals 1 (from (1) and (2)) and Langevin thermostat temperature T* = k_B T = e^2/(4 pi eps_0) 1/U 1/R = e^2/(4 pi eps_0) = 1.44e-9, but it doesnt give the same result for energy compared to MC. What do you think?

Grigory Zarubin
Abt. Dietrich
room 5E16
+49 173 8565123

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