- ID, group-ID are documented in compute command +
- stress/atom = style name of this compute command + +
- zero or more keyword/arg pairs may be appended + +
- keyword = ke or pair or bond
+
+
ke arg = yes or no + yes/no = include/exclude kinetic energy contribution to stress + pair arg = yes or no + yes/no = include/exclude pairwise energy contribution to stress + bond arg = yes or no + yes/no = include/exclude bond energy contribution to stress +
+
Examples:
compute 1 mobile stress/atom+
compute 1 all stress/atom ke no +
Description:
Define a computation that computes the per-atom stress tensor for each atom in a group. The 6 components can be output via the dump custom command.
-The stress tensor is computed for only pairwise forces where the ab -component of stress on atom i is given by +
The stress tensor for each atom is the sum of 3 terms in the following +formula. Any of the terms can be excluded by setting the ke, +pair, or bond keywords to no. The ab component of stress on +atom I is as follows, where a and b take on values x,y,z to +generate the 6 components of the symmetric tensor:
where the first term is a kinetic energy component for atom i, j -loops over the N neighbors of atom i, and Fb is one of 3 -components of force on atom i due to atom j. Both a and b -take on values x,y,z to generate the 6 components of the symmetric -tensor. +
The first term is a kinetic energy contribution for atom I. The +second term is a pairwise energy contribution where J loops over the +Np neighbors of atom I, and Fp is one of 3 components of force +on atom I due to the pairwise interaction with atom J. The third +term is a bond energy contribution where J loops over the Nb bonds +which atom I is part of, and Fb is one of 3 components of force on +atom I due to the bonded interaction with atom J.
-Note that this formula for stress does not include virial -contributions from intra-molecular interactions (e.g. bonds, angles, -torsions, etc). Also note that this quantity is the negative of the +
If two atoms interact, either pairwise or in a bond, and only one of +them is in the compute group, the interaction is included in the +per-atom stress of the atom in the group. +
+For force fields that include a contribution to the pairwise +interaction that is computed as part of dihedral terms (i.e. 1-4 +interactions), this contribution is NOT included in the per-atom +pairwise stress. +
+IMPORTANT NOTE: The per-atom stress does NOT include contributions due +to angles, dihedrals, impropers that the atom is part of, or a +long-range Coulombic contribution. Currently, there is no way in +LAMMPS to calculate per-atom stress from angles, dihedrals, improper, +or long-range interactions. +
+Note that as defined above, per-atom stress is the negative of the per-atom pressure tensor. It is also really a stress-volume formulation. It would need to be divided by a per-atom volume to have units of stress, but an individual atom's volume is not easy to compute in a deformed solid. Thus, if you sum the diagonal components of the per-atom stress tensor for all atoms in the system and divide the sum by 3V, where V is the volume of the system, you should get -P, -where P is the total pressure of the system. +where P is the total pressure of the system (assuming there is no +angle, dihedral, improper, or long-range contribution to the total +pressure).
Computation of per-atom stress tensor components requires a loop thru -the neighbor list and inter-processor communication, so it can be -inefficient to compute/dump this quantity too frequently or to have -multiple compute/dump commands, each of a stress/atom style. +the pairwise and bond neighbor lists and inter-processor +communication, so it can be inefficient to compute/dump this quantity +too frequently or to have multiple compute/dump commands, each of a +stress/atom style.
-Restrictions: none +
Restrictions: +
+Some pair potentials do not allow the calculation of per-atom stress +via this command. An error will be generated if this is the case.
Related commands: none
-Default: none +
Default: +
+The option defaults are ke = yes, pair = yes, bond = yes.