Code review comment for lp:~nickpapior/siesta/low-D-stress

A couple of preliminary questions:

1. Are the resulting 1D and 2D stresses vacuum independent now?
2. At least for surfaces, a physically meaningful concept is that of the
'surface stress', for which a computational recipe exists. I do not think
that your 2D stress will map to that in the case of a slab meant to
represent a surface. The concept of surface stress does not apply to
one-atomic-layer materials, however.

On Thu, Feb 14, 2019, 11:05 Nick Papior <<email address hidden> wrote:

> Nick Papior has proposed merging lp:~nickpapior/siesta/low-D-stress into
> lp:siesta/4.1.
>
> Commit message:
> Added 1D and 2D stress tensor calculations
>
> The problem with low-D materials is that stress is "per volume",
> but since vacuum is independent in a slab calculation the stress will
> be variable depending on the amount of vacuum.
>
> What one *really* should do is to use the volume of the slab along the
> vacuum direction so it may be fixed depending on the extend of the slab.
> This poses additional problems since how do you decide the slab thickness?
> - vdW bond-lengths
> - orbital radii
> - or 3rd?
>
> This amends output to do the simplest thing but forces users to do
> post-processing.
> The vacuum region is divided out and thus change the units depending on
> the dimensionality.
> - 2D == eV/Ang^2
> - 1D == eV/Ang
>
> Then the user can them-selves determine the actual height/area of the
> slab/chain.
>
> Note that the regular stress tensors are still printed out, but now an
> additional
> block is added.
>
> Requested reviews:
> Alberto Garcia (albertog)
>
> For more details, see:
> https://code.launchpad.net/~nickpapior/siesta/low-D-stress/+merge/363189
>
> In addition to the above message I would like to discuss the following
> which came up during the coding:
>
> 1) I am currently using nsc to determine the periodic directions, not
> ideal but I put in this remark in the code:
> ! TODO, probably we need to use shaper...
> ! However, the shaper returns vectors which are
> ! not parallel to the actual lattice vectors and hence
> ! it becomes difficult to utilize them
> ! What to do for Gamma calculations?
>
> I tried using shaper, to no avail as noted above...
>
>
> 2) Regarding forces in the write_forces. Currently ghost atoms are also
> considered in sum(fa) and res(fa), don't you agree we need to change that
> to not include ghost atoms (at least in the residual?) Or?
> --
> You are requested to review the proposed merge of
> lp:~nickpapior/siesta/low-D-stress into lp:siesta/4.1.
>