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Limit atom energy change in fix dt/reset #933

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merged 2 commits into from Jun 18, 2018

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rtoijala commented May 28, 2018

Purpose

Allow limiting of the maximum energy change of an atom in fix dt/reset in addition to the existing distance limit. Useful especially for high-energy irradiation.

Author(s)

Risto Toijala @ University of Helsinki (only this implementation)
Kai Nordlund @ University of Helsinki (original implementation in PARCAS)

Backward Compatibility

No backwards incompatible changes. The energy change limit itself has been in use in the PARCAS MD code for years.

Implementation Notes

Minimal changes. Everything works as before unless the new feature is used.
No backwards incompatible changes.

Post Submission Checklist

I am unsure how to interpret the title above. Should this be filled by me when submitting, or by other developers when reviewing?

Please check the fields below as they are completed

  • The feature or features in this pull request is complete
  • Suitable new documentation files and/or updates to the existing docs are included
  • One or more example input decks are included
  • The source code follows the LAMMPS formatting guidelines

Further Information, Files, and Links

Original article (see section 4.1): http://www.acclab.helsinki.fi/~knordlun/pub/Nor94b.pdf

rtoijala and others added some commits May 23, 2018

Limit atom energy change in fix dt/reset
Allow limiting of the maximum energy change of an atom in
fix dt/reset in addition to the existing distance limit.
Useful especially for high-energy irradiation.
clarify docs on what energy is monitored
changes to the energy only consider the kinetic energy, so make that explicit in the augmented `fix dt/reset` docs

@akohlmey akohlmey self-assigned this May 31, 2018

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akohlmey commented May 31, 2018

@rtoijala thanks for your contribution to LAMMPS. This looks nice and is straightforward to integrate. I've made a small tweak to the docs to clarify which energy exactly is monitored.

@akohlmey akohlmey assigned sjplimp and unassigned akohlmey May 31, 2018

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rtoijala commented Jun 18, 2018

Hey, is this waiting for me to do something?

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sjplimp commented Jun 18, 2018

Nice option to add to fix dt/reset - thanks for contributing it. I'm somewhat unclear from the doc page, if the new option is imposed in addition to the Xmax criterion, or meant to be allowed as an alternative?
From a user standpoint, Is it useful to have either or both criteria (xmax, emax) ?

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rtoijala commented Jun 18, 2018

Yes, the new option is meant to be imposed in addition to the Xmax criterion. As a user, I would expect both criteria to be checked. The fix is meant to limit the timestep to a value that does not lead to unphysical results, and both long distances and large changes in energy can lead to problems. I cannot imagine anyone (at least at our lab) turning of either criterion when performing irradiation simulations.
I am of course open to making it a user choice whether the energy criterion is applied additionally or alternatively.
Do you have suggestions for better wording of the documentation? Would an added sentence "When enabled, the kinetic energy criterion is applied in addition to the maximum distance criterion." be sufficient?

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sjplimp commented Jun 18, 2018

Do you have suggestions for better wording of the documentation? Would an added sentence "When >enabled, the kinetic energy criterion is applied in addition to the maximum distance criterion." be >sufficient?

yes, something like that sentence plus: In other words the new timestep will be small enough to meet both criteria.

If you change the doc file, I'll approve the PR - thanks!

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sjplimp commented Jun 18, 2018

actually, I can do the doc change - I need to do some other doc edits

@sjplimp sjplimp merged commit 03a7d1c into lammps:master Jun 18, 2018

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@rtoijala rtoijala deleted the rtoijala:fix_dt_reset_energy branch Jun 18, 2018

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