questions on pair AGNI

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1

Dear LAMMPS users and developers,
I have few questions regarding the implementation of pair AGNI. I use LAMMPS version 22Aug18.

  1. According to the documentation, this potential does not provide access to stress. I do not understand why because it seems that pressure in LAMMPS is computed from the force and position on the atoms? Moreover, when I run the example provided with the pair style, it displays pressure.
  2. In the implementation (pair_agni.cpp) the fingerprint is computed as exp(-eta*r2) whereas in the papers (JPCC, PRB and IJQC) all show an exp(-(r/eta)^2). Is there any reason for that?
  3. In the calculation of the force lines 216-218 of the pair_agni.cpp file, there is an additional term iparam.b which is added. This does not appear anywhere in the papers… Any reason for this extra term?
  4. Finally, it seems that the potential conserves well energy in the micro canonical ensemble. How the model guaranties that the sum of all forces add up to zero (and therefore conserve energy) since it predicts an approximation of the local force on each atom? After checking the sum of the forces its seems to be rather different than zero but well constant.
    Thank you,
    regards bob.
2

Dear LAMMPS users and developers,
I have few questions regarding the implementation of pair AGNI. I use LAMMPS version 22Aug18.

  1. According to the documentation, this potential does not provide access to stress. I do not understand why because it seems that pressure in LAMMPS is computed from the force and position on the atoms? Moreover, when I run the example provided with the pair style, it displays pressure.

how (global) pressure is computed in LAMMPS depends on the individual force styles. in some cases, F dot r is used, but not for all. pair style agni uses a non-standard way to tally stress, that has not been validated by the original authors of the pair style.

the remaining questions you best ask the folks that authored the papers directly.

axel.

3

Hello,

I am one of the original authors of the AGNI methodology; I figured I answer your questions here since some of them are relevant to LAMMPS.