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Re: [ESPResSo-users] minimum number of cells

From: Xikai Jiang
Subject: Re: [ESPResSo-users] minimum number of cells
Date: Thu, 17 Jul 2014 14:23:47 -0400

Dr. Arnold:

Yes, it's an error, not warning, for "number of cells is smaller than minimum". The simulation box sizes are 20, 20 and 19 and each side is larger than twice the r_cut. P3M parameters are from automatic tuning.

It requires at least 2 cells per spatial dimension, then "min_num_cells" is 8 on 1 core, and it's 1 on 8 cores. For 16 cores, if I set min_num_cells to 1, for the whole system, the total min_num_cells is 1*16=16 and it's safe for the simulation, is my understanding correct?

Thank you four your time and help.


Date: Wed, 16 Jul 2014 22:12:01 +0200
From: address@hidden
To: address@hidden; address@hidden
Subject: Re: [ESPResSo-users] minimum number of cells

On 16.07.14 18:59, Xikai Jiang wrote:
Dear all:

I have a question about the global variable "minimum_num_cells".

Espresso seems to set "minimum_num_cells" to 8 by default, but
when I use MPI and use more cores, it gives me a warning saying
"number of cells is smaller than minimum". And I found when I
increase number of cores, number of cells decreases.

You don't get a warning, but an error message, since Espresso would produce incorrect results. We need at least two cells per spatial dimension to make the minimum image convention unambiguous. minimum_num_cells however counts per core, so if you have 2 cores, you just need 4, and for 4 cores 2.

I'm using P3M algorithm and Coulomb cutoff is 3.3:
inter coulomb [expr 138.935485/$epsilonr/$T] p3m 3.3 100 7 1.1

Here I'm supposed not to change the interaction range (r_cut) and
my question is, is it safe to manually set "minimum_num_cells" to 0,
in order to avoid the warning and let me use more cores?

No, first of all you cannot set minimum_num_cells to a lower value, since ESPResSo wants to avoid computing incorrect results. As a consequence, there is a limit to the cutoff depending on the box size. You simply cannot use r_cut 3.3 in a box smaller than 6.7. Either, you put more particles to increase the box size, or you don't insist on your parameters, but rather let P3M tune them.


JP Dr. Axel Arnold
ICP, Universit├Ąt Stuttgart
Allmandring 3
70569 Stuttgart, Germany
Email: address@hidden
Tel: +49 711 685 67609

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