Phonon Optimized Potentials

If you are fitting a tabulated potential, this file is used to declare which parameters are being fit.

Fitting the Tersoff potential, for example, yields the following REFPOT:

# Example of a Tersoff potential file for fitting
#   m, Inmma, lambda3, c, d, costheta0, n, beta, lambda2, B, R, D, lambda1, A
Ge Ge Ge 3 1  [] [] [] [] [] [] [] [] 2.95  0.15 [] []

where the “[]” brackets designate parameters that are to be fit.

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SYMMETRIC PARAMETERS

It is of use to define symmetries between parameters at times. If you want all of a certain parameter to be the same, use [1] for that set of parameters. Use [2] for the next set of symmetric parameters, etc..

An example is shown here:

Ge Ge Ge 3 1  [1] [2] [3] [4] [] [] [] [] 2.95  0.15 [] []
Ge Ge Si 3 1  [1] [2] [-3] [-4] [] [] [] [] 2.95  0.15 [] []

All the parameters with a number in the “[]” brackets will be treated as a single parameter during fitting.

The “[]” brackets with negative numbers will adopt the same magnitude of the parameters in [3] and [4], but with opposite signs. This is useful when fitting charges.