[lammps-users] eFF and data files

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1

Dear developper,
I am trying using eFF forcefields. I am reading the manual and the and the cited Jaramillo-Botero article. Also the peFF related presentation at sandia in 2010.
I have a trouble with electron definition in the data file. see for example the data.CH4 in /potentials/USER/eff/CH4 folder.
At each H atom position there are 2 electron of spin 1 and -1 and the electron charge is 0.
And when running the job there are in the dump…xyz file 2 electron per H atom.
I do not understand why we should put 2 electrons in the data file : Is there some random selection of spin ? Why the charge is 0 for e- since it is 6 for the Carbon and 1 for H.
when reading the article I did not find something related to this. May be there is another reference detailing this ?
thanks a lot for your help.

The data file is below

pascal

Created by AJB

15 atoms
3 atom types

-500.0 500.0 xlo xhi
-500.0 500.0 ylo yhi
-500.0 500.0 zlo zhi

Masses

1 12.01070 # C nuclei mass
2 1.000794 # H nuclei mass
3 1.000000 # electron mass

Atoms
#id|type|q|spin|eradius|x|y|z|
1 1 6.0 0 0.0 0.0 0.0 0.0
2 2 1.0 0 0.0 1.0 1.0 1.0
3 2 1.0 0 0.0 -1.0 -1.0 1.0
4 2 1.0 0 0.0 1.0 -1.0 -1.0
5 2 1.0 0 0.0 -1.0 1.0 -1.0
6 3 0.0 -1 0.5 0.0 0.0 0.0
7 3 0.0 -1 1.0 1.0 1.0 1.0
8 3 0.0 -1 1.0 -1.0 -1.0 1.0
9 3 0.0 -1 1.0 1.0 -1.0 -1.0
10 3 0.0 -1 1.0 -1.0 1.0 -1.0
11 3 0.0 1 0.5 0.0 0.0 0.0
12 3 0.0 1 1.0 1.0 1.0 1.0
13 3 0.0 1 1.0 -1.0 -1.0 1.0
14 3 0.0 1 1.0 1.0 -1.0 -1.0
15 3 0.0 1 1.0 -1.0 1.0 -1.0

2

I don’t have any experience with eFF simulations, but your observations can be easily rationalized.

  • the provided charge information is obviously the “core charge”. Electrons don’t have an atom core but also their charge is defined by physics and thus not really needed.
  • eFF apparently prefers to associate both atoms in a (sigma) bond with the same atom and thus the hydrogen atom would be the natural choice. Due to the Pauli principle, those electrons have to have opposite spin.
3

Thanks a lot Axel,
I thin I understand what you mean. " eFF apparently prefers to associate both atoms in a (sigma) bond with the same atom and thus the hydrogen atom would be the natural choice. "
Best
Pascal