Unit system
| Quantity | Unit | Notes |
|---|---|---|
| Energy | eV | Electron volts. |
| Forces | eV/Å | Per-atom 3-vector. |
| Distances / positions | Å | Angstroms. |
| Stress | eV/ų | Voigt notation. |
| Temperature | K | Kelvin. |
| Time | fs | Femtoseconds; converted internally via ase.units.fs. |
| Pressure | GPa (user-facing) | 1 GPa = 10,000 bar. Internally represented as eV/ų. |
Conversion at boundaries
The same physical quantity can appear in slightly different forms as it moves from your input, through ASE and MACE, into the JSON result, and finally onto the screen. The table below tracks each quantity across those boundaries.
| Boundary | Energy | Forces | Distance | Time | Temperature | Pressure |
|---|---|---|---|---|---|---|
| User input | — | — | Å | fs | K | GPa |
| ASE internal | eV | eV/Š| Š| ASE units | K | eV/ų |
| MACE output | eV | eV/Å | Å | — | — | — |
| JSON result | eV | eV/Å | Å | seconds | — | — |
| Frontend display | eV | eV/Å | Å | seconds | K | GPa |
Critical conversion: pressure
Pressure entered in GPa is converted to ASE’s internal bar units aspressure_GPa × 1e4 = pressure_bar. This conversion only applies at the NPT input boundary — internally, ASE and MACE both work in eV/ų.Common pitfalls
- Never mix units. MACE outputs eV/Å, ASE uses eV/Å internally, and the frontend displays eV/Å — there is no silent conversion between them anywhere in the pipeline.
- Reference energy keys vary. Extended XYZ metadata can store a reference energy under
REF_energy,ref_energy,energy, ordft_energy— check which key a given file uses before comparing to a calculated value. - Benchmark catalog values are experimental, not DFT. Reference energies in the built-in catalog come from experimental sources, not density functional theory, so comparisons against them should note the difference in level of theory.
- MACE-MP-0 and MACE-OFF use different energy-reference conventions. Their absolute energies differ by roughly an order of magnitude and are not directly comparable.
Do not compare absolute energies across models
MACE-MP-0 (−1 to −15 eV/atom) and MACE-OFF (−100 to −600 eV/atom) use different isolated-atom energy references. Never compare their absolute energies directly — compare energy differences computed with the same model instead. See Foundation models for the full energy-reference conventions.