# compute stress/tally command

## Syntax

compute ID group-ID style group2-ID

• ID, group-ID are documented in compute command

• style = force/tally or pe/tally or pe/mol/tally or stress/tally

• group2-ID = group ID of second (or same) group

## Examples

compute 1 lower force/tally upper
compute 1 left pe/tally right
compute 1 lower stress/tally lower


## Description

Define a computation that calculates properties between two groups of atoms by accumulating them from pairwise non-bonded computations. The two groups can be the same. This is similar to compute group/group only that the data is accumulated directly during the non-bonded force computation. The computes force/tally, pe/tally, stress/tally, and heat/flux/tally are primarily provided as example how to program additional, more sophisticated computes using the tally callback mechanism. Compute pe/mol/tally is one such style, that can - through using this mechanism - separately tally intermolecular and intramolecular energies. Something that would otherwise be impossible without integrating this as a core functionality into the based classes of LAMMPS.

The pairwise contributions are computing via a callback that the compute registers with the non-bonded pairwise force computation. This limits the use to systems that have no bonds, no Kspace, and no many-body interactions. On the other hand, the computation does not have to compute forces or energies a second time and thus can be much more efficient. The callback mechanism allows to write more complex pairwise property computations.

Output info:

Compute pe/tally calculates a global scalar (the energy) and a per atom scalar (the contributions of the single atom to the global scalar). Compute pe/mol/tally calculates a global 4-element vector containing (in this order): evdwl and ecoul for intramolecular pairs and evdwl and ecoul for intermolecular pairs. Since molecules are identified by their molecule IDs, the partitioning does not have to be related to molecules, but the energies are tallied into the respective slots depending on whether the molecule IDs of a pair are the same or different. Compute force/tally calculates a global scalar (the force magnitude) and a per atom 3-element vector (force contribution from each atom). Compute stress/tally calculates a global scalar (average of the diagonal elements of the stress tensor) and a per atom vector (the 6 elements of stress tensor contributions from the individual atom).

Both the scalar and vector values calculated by this compute are “extensive”.

## Restrictions

This compute is part of the USER-TALLY package. It is only enabled if LAMMPS was built with that package. See the Build package doc page for more info.

Not all pair styles can be evaluated in a pairwise mode as required by this compute. For example, 3-body and other many-body potentials, such as Tersoff and Stillinger-Weber cannot be used. EAM potentials only include the pair potential portion of the EAM interaction when used by this compute, not the embedding term. Also bonded or Kspace interactions do not contribute to this compute.

The computes in this package are not compatible with dynamic groups.