Conventions

Data Format Conventions

• Cell vectors are given in ASE style as the rows of the cell matrix

Pressure / stress / virials

nequip always expresses stress in the “consistent” units of energy / length³, which are not the typical physical units used by many codes for stress. For example, consider data where the cell and atomic positions are provided in Å and the energy labels are in eV but the stress is given in GPa or kbar. The user would then need to convert the stress to eV/ų before providing the data to nequip.

Warning

Training labels for stress in the original dataset must be pre-processed by the user to be in consistent units.

Stress also includes an arbitrary sign convention, for which we adopt the choice that virial = -stress x volume  <=>  stress = (-1/volume) * virial. This is the most common sign convention in the literature (e.g. adopted by ASE), but notably differs from that used by VASP (see here). In the NequIP convention, stress is defined as the derivative of the energy \(E\) with respect to the strain tensor \(\eta_{ji}\), divided by the volume \(\Omega\):

\[\sigma_{ij} = \frac{1}{\Omega} \frac{\delta E} {\delta \eta_{ji}}\]

Positive diagonal entries of the stress tensor imply that the system is under tensile strain and wants to compress, while negative values imply that the system is under compressive strain and wants to expand. When VASP results are parsed by ASE, the sign is flipped to match the nequip convention.

If your dataset uses the opposite sign convention, either preprocess the dataset to be in the NequIP stress sign convention, or use the StressSignFlipTransform data transform.

Users that have data with virials but seek to train on stress can use the data transform VirialToStressTransform.