ediff.in

This file lists linear combinations of the total energies of structures, such as energy differences and formation energies, that enter the loss function through the keyword lambda_d. It is read only when the keyword lambda_d is set.

File format

Each line holds one combination:

[+|-] <term> {+|- <term>} [w=<weight>]

• A term is either <name> or <coefficient>*, without spaces. name is the label given by the name= field on the comment line of a structure in train.xyz and test.xyz. Names are matched case-insensitively for the letters A to Z, and a name may occur only once per line. coefficient is a nonzero real number or a fraction p/q of two real numbers, such as 1/3, and defaults to 1.

• The operators + and - are fields of their own, separated from the terms by whitespace.

• w= is optional and comes last. weight must be a positive number and defaults to 1. It scales the contribution of the combination to the loss.

A combination needs at least two structures and must be balanced in the number of atoms of each type: for every type, the sum of the coefficients times the numbers of atoms of that type vanishes. The check allows \(10^{-6}\) atoms of each type to be left over, so a coefficient such as one third is best written as the fraction 1/3. No two structures of a combination may be the same structure for the model, that is have the same types, the same positions to \(10^{-5}\) Å, the same boundaries, for periodic boundaries the same cell, and for model_type 3 the same temperature. Two such structures can differ at most in charge, which a NEP model ignores and for which a qNEP model predicts no meaningful energy difference. Atoms are compared in order, so a copy with reordered atoms or with an atom shifted by a lattice vector is not detected. The target of a combination is the same combination of the target total energies, which are given by the energy fields of the structures.

A field beginning with # starts a comment that extends to the end of the line. A line that breaks any of these rules is an input error.

Example

# oxygen vacancy formation energy against the perfect cell and the O2 molecule
vac_O + 1/2*O2 - ideal
# migration barrier of the oxygen vacancy, from the saddle point against the minimum, with weight 2
vac_O_saddle - vac_O w=2
# vacancy formation energy in a 31-atom cell against the 32-atom perfect cell of one element
vac31 - 31/32*bulk32

Training and test combinations

A combination with all its structures in train.xyz is a training combination, which enters the loss function and the column rmse_ediff_train of loss.out. A combination with all its structures in test.xyz is a test combination, which enters the column rmse_ediff_test. A combination whose names occur in both files is both a training and a test combination. A combination with a name that occurs in neither file, or with its structures in different files, is skipped. nep prints the number of combinations of each kind and a warning with the number of skipped combinations.