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# Contributors:
#    Ryan S. Elliott
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#    Valeriu Smirichinski
#    Hao Xu


This directory contains a Morse pair potential Model driver written in C.

   phi(r) = epsilon * ( - exp[ -2*C*(r - Rzero) ] + 2*exp[ -C*(r - Rzero) ] )
                    * S(r, logisticWidth, logisticR0)

where S(r, logisticWidth, logisticR0) is the logistic function.

   S(r, logisticWidth, logisticR0)
     = 1/(1 + exp( -logisticWidth*[r - logisticR0]))

The choice of

   logisticR0 = cutoff + 7/logisticWidth

is designed so that S(cutoff) ~ 0.001.

This model driver publishes its parameters, and supports optional
computation of `energy', `forces', `particleEnergy', `process_dEdr', and
`process_dE2dr2'.

To create a KIM Model from this Model Driver, a parameter file is required.
This file must have the following format:
   Line 1: Species name
   Line 2: `cutoff' value in Angstroms
   Line 3: `logisticWidth' value as pure number > 0.0
   Line 4: Morse `epsilon' value in eV
   Line 5: Morse `C' value in 1/Angstroms
   Line 6: Morse `Rzero' value in Angstroms
Any additional lines will be silently ignored.