Hi all!
Happy New Year!
In some cases it makes sense to store the particle coordinates in
absolute
coordinates in a periodic system even when they are outside the primary
box
(for example when tracking the MSD of the particle). In that case 'image'
would not be defined.
Also, it might make sense to store coordinates outside the primary box
even
when 'image' is used (for example, it is common practice that particles
can
walk out of the box up to skin/2 before the image is updated for
performance reasons.
To make that clear, I propose to change the last paragraph (starting with
"For instance...") with the following paragraph:
If the component $d$ of `box/boundary` is set to "none" (see below)
and
`image` is set, the value of the corresponding component $d$ of `image`
has
to be silently ignored. If the component $d$ of `box/boundary` is set to
"periodic", this does neither mean that the element `image` has to exist
nor that the position has to be within the primary box. The component $d$
of the absolute position of particle $i$ is then computed as $R_{id} =
r_{id} + L_d a_{id}$, where $\vec r_i$ is taken from `position`, $\vec
a_i$ is taken from `image`, and $\vec L$ from `box/edges`.
Furthermore, I would propose to extend the paragraph "Simulation box" ->
"boundary" as follows:
For those components where `boundary` is set to "none", any value of
`edges` or `offset` must be silently ignored.
Finally, I am still wondering about the meaning of the `offset` field,
given that we give no guarantees on the limits of the positions. What
could
anybody do with this value?
Olaf