Re: [ESPResSo-users] Temperature units in Langevin thermostat

[Peter Košovan]

Hi Grigory,

if I am not mistaken, your e-mail has not been answered yet. Note that the Bjerrum length parameter of electrostatics and the Temperature parameter of the Langevin are independent input parameters, although physically they both depend on temperature.

If you are indeed using hard spheres (not LJ spheres), then you cannot use the Langevin MD due to infinite forces. In such case, you need not worry about Langevin temperature. On the other hand, if you are using the LJ potential to simulate "quite hard" spheres, then you need to specify additional parameter, which defines the LJ interaction strength (epsilon) in terms of kT, and its relation to the electrostatic interaction parameter U. This will immediately define the Langevin temperature parameter.

With regards,

peter

2014-09-10 15:20 GMT+02:00 Grigory Zarubin <address@hidden>:

Hello,
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
MPI IS
Abt. Dietrich
room 5E16
+49 173 8565123
*****************

--

Dr. Peter Košovan

Departmtent of Physical and Macromolecular Chemistry

Faculty of Science, Charles University in Prague, Czech Republic

Katedra fyzikální a makromolekulární chemie

Přírodovědecká fakulta Univerzity Karlovy v Praze

www.natur.cuni.cz/chemistry/fyzchem/
Tel. +420221951290

Fax +420224919752

---

Pokud je tento e-mail součástí obchodního jednání, Přírodovědecká fakulta Univerzity Karlovy v Praze:

a) si vyhrazuje právo jednání kdykoliv ukončit a to i bez uvedení důvodu,

b) stanovuje, že smlouva musí mít písemnou formu,

c) vylučuje přijetí nabídky s dodatkem či odchylkou,

d) stanovuje, že smlouva je uzavřena teprve výslovným dosažením shody na všech náležitostech smlouvy.