maint(kokkosNL): nicer failure modes for large cutoffs in dense systems

[HaoZeke]

Summary

• Detect when estimated neighbor count for a metatomic model cutoff exceeds neigh_modify one and error with a message specifying the exact neigh_modify line to add
• Add defensive guards in npair_kokkos and nbin_kokkos to produce clear error messages instead of SIGFPE when bin parameters are degenerate
• Generalize the fix_metatomic binsize hack to apply whenever the user has not explicitly set binsize (not just when pair_style none)

Details

Dense systems with large cutoffs (e.g. PET-MAD with ~5.5 A interaction range) crash with SIGFPE in NPairKokkosBuildFunctorGhost. Root causes:

1. Default one=2000 (max neighbors per atom) is too small for dense systems with large cutoffs
2. pair_metatomic did not set binsize, and fix_metatomic only set it when pair_style none
3. The Kokkos NL builder has no guard against degenerate bin parameters

Changes:

• pair_metatomic::init_style: estimate neighbors from (4/3)*pi*(cutoff+skin)^3 * density with 2x safety margin; error if this exceeds neigh_modify one, with a message specifying the exact fix
• fix_metatomic::init: generalize binsize hack to apply whenever binsizeflag is unset; add the same density-based neighbor check with clear error
• npair_kokkos.cpp: guard atoms_per_bin <= 0 before the build while-loop
• nbin_kokkos.cpp: guard mbins <= 0 in bin_atoms_setup

Example error message:

The metatomic model cutoff (5.5000) with current system density requires
approximately 4200 neighbors per atom, but neigh_modify one is only 2000.
Add 'neigh_modify one 4200 page 42000 binsize 2.7500' to your input script.

Changes:

• pair_metatomic::init_style: auto-set binsize = 0.5 * max_cutoff when user has not set binsizeflag; estimate neighbors from (4/3)*pi*(cutoff+skin)^3 * density with 2x safety margin; auto-increase one/page with warning
• fix_metatomic::init: generalize binsize hack to apply whenever binsizeflag is unset (not just pair_style none); add same density-based one/page auto-increase
• npair_kokkos.cpp: guard atoms_per_bin <= 0 before the build while-loop
• nbin_kokkos.cpp: guard mbins <= 0 in bin_atoms_setup

User workaround (still works as override): neigh_modify one 100000 page 1000000 binsize 5.5

cc @PicoCentauri

Related Issue(s)

Internal comments.

Author(s)

Licensing

By submitting this pull request, I agree, that my contribution will be included in LAMMPS and redistributed under either the GNU General Public License version 2 (GPL v2) or the GNU Lesser General Public License version 2.1 (LGPL v2.1).

Artificial Intelligence (AI) Tools Usage

By submitting this pull request, I confirm that I did NOT use any AI tools to generate
all or parts of the code and modifications in this pull request.

Backward Compatibility

Implementation Notes

Post Submission Checklist

• The feature or features in this pull request is complete
• Licensing information is complete
• Corresponding author information is complete
• The source code follows the LAMMPS formatting guidelines
• Suitable new documentation files and/or updates to the existing docs are included
• The added/updated documentation is integrated and tested with the documentation build system
• The feature has been verified to work with the conventional build system
• The feature has been verified to work with the CMake based build system
• Suitable tests have been added to the unittest tree.
• A package specific README file has been included or updated
• One or more example input decks are included

Further Information, Files, and Links

[PicoCentauri]

LGTM