Dear all

I need to calculate the local stress in a group during loading and I used the stress/atom and compute reduce commands.

to calculate the real stress I need to use the volume of group. I have searched the mailing list and find that there is not a direct command for do this.

1- is there a way to compute the real stress in group without compute the local volume, because I just need it to compute the stress?

2- is there a method to compute the volume of a group?

thanks

Dear all

I need to calculate the local stress in a group during loading and I used

the stress/atom and compute reduce commands.

to calculate the real stress I need to use the volume of group. I have

searched the mailing list and find that there is not a direct command for

do this.

...and there is a good reason for that: the volume of a single atom is

an ill-defined property. you can only infer an effective volume

indirectly, e.g. via density.

1- is there a way to compute the real stress in group without compute the

local volume, because I just need it to compute the stress?

if you want spatially resolved stress, you might find help in the

atoms-to-continuum (USER-ATC) package. other than that, you probably

will have to search the literature for a derivation (or come up with

one by yourself if there isn't any) and then program it.

2- is there a method to compute the volume of a group?

not really. it is not a well defined property. you could make some

assumptions, but they will all have flaws. if you'd assume an

effective volume per atom and sum over that, you cannot account for

volume changes due to changes in packing. if you use some kind of

tessellation, you have to make an assumption about the radius of an

atom, since the simulation itself uses point particles which have no

volume (and radius) at all. and so on...

axel.

While the volume of a single atom is not well defined, the volume of a

compact group (cluster) is (the larger the cluster more meaningful is

the volume). LAMMPS has a computer to calculate the voronoi volume per

atom. You can combine that with a "reduce sum" compute to obtain the

total voronoi volume of the group.

Daniel beat me to this response (and was too modest to mention

that he wrote the new compute voronoi/atom command).

This is a great use for the new command, and is probably the

best conceptual way to get the "volume" of a group of atoms for the

stress calculation you are doing.

Daniel - can't remember if you ever told me if it is possible

to use the Voro++ package (that the compute wraps) to get

voronoi areas for 2d systems?

Steve

Daniel - can't remember if you ever told me if it is possible

to use the Voro++ package (that the compute wraps) to get

voronoi areas for 2d systems?

I do not think so. But I could take a look at using a different

backend such as "Qhull" or "Viennagrid", which also do 2d.

Such a change would be transparent to the LAMMPS users.