Conventions and units

Conventions

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

  • The first index in an edge tuple (edge_index[0]) is the center atom, and the second (edge_index[1]) is the neighbor

Units

nequip has no prefered system of units; models, errors, predictions, etc. will always be in the units of the original dataset used.

Warning

nequip cannot and does not check the consistency of units in inputs you provide, and it is your responsibility to ensure consistent treatment of input and output units

Losses (training_loss_f, validation_loss_e, etc.) do not have physical units. Errors (training_f_rmse, validation_f_rmse) are always reported in physical units.

Pressure / stress / virials

nequip always expresses stress in the “consistent” units of energy / length^3, which are not the typical physical units used by many codes for stress.

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.

Warning

Training labels for stress in the original dataset must be pre-processed by the user to be in this sign convention, which they may or may not already be depending on their origin.