
From:  Xikai Jiang 
Subject:  Re: [ESPResSousers] 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. Xikai Date: Wed, 16 Jul 2014 22:12:01 +0200 From: address@hidden To: address@hidden; address@hidden Subject: Re: [ESPResSousers] minimum number of cells On 16.07.14 18:59, Xikai Jiang wrote:
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.
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. Best, Axel  JP Dr. Axel Arnold ICP, Universität Stuttgart Allmandring 3 70569 Stuttgart, Germany Email: address@hidden Tel: +49 711 685 67609 
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