Question regarding fix_modify atc

Dear Lammps users,

I have some questions regarding fix_atc with hardy type. I am trying to calculate stresses in a cluster (large) of atoms and I decided to use the hardy command to map the atomic stresses onto a mesh. I recently read the paper “Calculation of stress in atomistic simulation” and I am trying to figure out how the method outlined in the paper can be implemented in LAMMPS.

I read some of the user examples on fix_atc hardy and I am using the following commands:

fix AtC all atc hardy
fix_modify AtC transfer internal type internal
fix_modify AtC fem create mesh 19 19 19 mdRegion f f f
fix_modify AtC transfer fields none
fix_modify AtC transfer fields add density stress
fix_modify AtC transfer set reference_potential_energy

My question is regarding setting the so-called “localization” function. Where in the code would I specify that the cubic function (with cutoff R_c) needs to be used? I am guessing this is the command to use:

fix_modify AtC transfer kernel quartic_sphere ${Ra}

where ${Ra} is the cutoff radius. Is this correct? Does this command correspond to the cubic localization function? Any suggestions are highly appreciated.

Thank you all for your time.

The AtC folks can answer this (CCd).


Hi Srikanth,

You’re correct, the command

fix_modify AtC transfer kernel quartic_sphere ${Ra}

specifies use of a spherical localization volume with a localization function that goes as (r/Ra)^4 [ with largest value at r = 0 and zero value at r=Ra ]. For a spherical volume with cubic dependence on (r/Ra),

fix_modify AtC transfer kernel cubic_sphere ${Ra}

There is documentation that goes with the user-atc package about what volume shapes and functions have been implemented.


Jon Z.

Thanks, Dr. Zimmerman. I found the doc files and they are really helpful. I was not aware that not all of them are online, thanks !!