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Re: [lammps-users] Different potential energy calculation results
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Re: [lammps-users] Different potential energy calculation results


From: Axel Kohlmeyer <akohlmey@...24...>
Date: Tue, 19 Dec 2017 16:55:27 -0500



On Tue, Dec 19, 2017 at 4:36 PM, Xiao Jia <xiaoj@...1508...> wrote:
Dear Dr. Kohlmeyer,

Sorry for the missing information. 
I am using the latest stable version of lammps, 11Aug2017. 
I tried another case with Morse potential, which generates consistent potential value to the log file.
While for EAM potential, it does not. 


​[...]​

I am not sure why different results can be generated for different potential, Morse and EAM,
and is there any difference to refer the same function single() by using different potentials.

​there is a very subtle difference between morse and eam. the former is a true pair-wise additive potential, the latter is not.
the PairEAM::single() function is dependent on position dependent properties in data structures, that are filled with suitable data during the original force computation.​

​do you get the same "wrong" number, when you are using compute group/group (which should be doing something similar to what you are doing with your modification?​

​axel.​

Thanks!


On Tue, Dec 19, 2017 at 9:45 AM, Axel Kohlmeyer <akohlmey@...29....> wrote:


On Tue, Dec 19, 2017 at 3:33 AM, Xiao Jia <xiaoj@...1508...> wrote:
Dear LAMMPS users,

I am referring the function single () defined in pair.cpp to calculate the pairwise force coefficient fpair and the potential energy.
eng=pair->single (i, j, itype, jtype, rsq, factor_coul, factor_lj, fpair);
The current potential I am using is EAM potential.

The weird observation I have is that my summation of the potential energy (eng value of each pair) from all the pairs does not match with the output from log file. The value I get is -21936114 and the value from log file is -63005273, the difference is close to 3 times. 

The calculation of pressure based on fpair seems to be consistent with the value from log file, but the difference in potential energy is hard to understand. 

​your e-mail is missing two important pieces of information.

1) which LAMMPS version you are using exactly. what you are reporting sounds a lot like a bug that was fixed not so long ago.
2) an example input deck that allows to quickly reproduce your issue (in case this is a real bug and still exists with the latest patch release).

axel.​


 

I will appreciate it so much if anyone can give me any idea about this issue.
Thanks!


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--
Dr. Axel Kohlmeyer  akohlmey@...24...  http://goo.gl/1wk0
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.




--
Dr. Axel Kohlmeyer  akohlmey@...24...  http://goo.gl/1wk0
College of Science & Technology, Temple University, Philadelphia PA, USA
International Centre for Theoretical Physics, Trieste. Italy.