Lammps ML-IAP interface with i-pi

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Kris Mu

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Jun 24, 2025, 1:42:14 AMJun 24

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Hi developers，
I’ve recently been trying to run MD simulations in LAMMPS using the ML-IAP interface to call a MACE model as an i-PI client, but I encountered a very strange issue: with the exact same input files, sometimes the simulation crashes immediately, while other times it runs to completion without error. When it fails, I see messages like:

```

ERROR on proc 0: Atoms have moved too far apart (-8.456443305474236e+252) for minimum image (src/MISC/fix_ipi.cpp:402)
```

By “sometimes,” I mean simply that I launch the jobs at different times; I make no changes to any input files between runs. Because it can succeed under identical conditions, the error is unlikely to be caused by my inputs. For comparison, I have also tested both the examples/clients/lammps example from the i-PI GitHub repository and a standalone LAMMPS + MACE ML-IAP run, and neither exhibits this problem.

All software versions and compilation options remain unchanged throughout:

LAMMPS source code version: 12 Jun 2025 (compile options attached)
i-PI version: 3.1.5
MACE version: 0.3.13

I have also attached my LAMMPS input file (which sometimes fails and sometimes succeeds). Any insight into why this intermittent “atoms moved too far apart” error occurs would be greatly appreciated.