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Re: [ESPResSo-users] (no subject)

From: Axel Arnold
Subject: Re: [ESPResSo-users] (no subject)
Date: Sun, 15 Mar 2015 20:56:53 +0100

Am 15.03.2015 um 16:24 schrieb Evyatar Arad <address@hidden>:

Hello again everyone,

Serval questions here.

1) While going through the code of the Velocity-Verlet integrator, I’ve noticed the 3rd integration step is preformed with a call to “force_calc”.

One of the comments describing the function stated that the forces are initialised with “friction_thermo_langevin” from the thermostat.cpp code ( "Initialise forces with: \ref friction_thermo_langevin (ghost forces with zero)”).

This may come as trivial, why are the langevin coefficients needed to initialise the basic particle forces (FENE, harmonic etc.)?

Who says so? What happens is that something needs to initialize the force. And since ever in Espresso the thermostat is the one that initializes the forces. The other forces like FENE or LJ are all added to this initial force.

2) Also, I couldn’t actually find any reference in the “force_calc” function’s body to “friction_thermo_langevin” or  “init_local_particle_force”. When and where does “friction_thermo_langevin” actually gets called?

force_calc calls init_forces, which in turn calls init_local_particle_force, which initializes the forces with the Langevin force from friction_thermo_langevin.

3)In addition, I’ve been inspecting propagate_vel_pos and noticed two main loops.

The first runs ‘local_cells.n' times and the secondary one runs 'cell->n' times.

What do these values mean?

The particles are organized in equally sized, cubic cells of size cell_size. local_cells is a list of pointers to the particle lists of those cells. Thus, local_cells.n = (box_l/cell_size)**3. In your case that seems to be 5 cells per side.

Now, within each cell, there are some particles, depending on the local density, so cell.n varies as particles move along the cells. The total sum of all particles in all cells stays the same, though.


Dr. Axel Arnold
ICP, Universität Stuttgart
Allmandring 3
70569 Stuttgart, Germany
Email: address@hidden
Phone: +49 711 685 67609

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