Dear All,
Yes, never fold back when studying the real mean-square displacement;
like Nicola wrote, your results would depend on the size of the box.
One issue to consider is whether one should remove the centre-of-mass
motion, as there is always such in DFT-based simulations (finite basis
set/grid in the exchange-correlation term, finite convergence toward
self-consistency, ...), unless it is removed manually (I do not know if
QE does it automatically). I tend to do it - sometimes the whole set of
atoms or molecules has travelled a long distance in a single direction.
Then there is also the question if the temperature during the simulation
is correct, or whether one should remove the centre-of-mass velocity also
at each step... Again, I do not know if QE handles this - can some one
confirm what is done (CPV, PW)?
Greetings from Sunny Paris,
apsi
-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-
Ari Paavo Seitsonen / [email protected] / http://www.iki.fi/~apsi/
Ecole Normale Supérieure (ENS), Département de Chimie, Paris
Mobile (F) : +33 789 37 24 25 (CH) : +41 79 71 90 935
On Mon, 30 Oct 2017, Marzari Nicola wrote:
You need to be able to follow genuine diffusion - eg a liquid, a superionic, a
fast interstitial...
If you fold, your msd will always tend to a constant (rather than eg a straight
line with finite slope, as a function of time).
Nic
Sent from a tiny keyboard... Contact info: http://theossrv1.epfl.ch/Main/Contact
On 30 Oct 2017, at 10:13, Lorenzo Paulatto <[email protected]> wrote:
On 30/10/17 10:09, Marzari Nicola wrote:
?? For MSD you should not fold back.
Re-reading the question i think you are right. But I also think that it
should not matter, i.e. that each atom should always stay close to its
initial position. If it matters the displacements are too large, maybe
the timestep is too large if doing molecular dynamics?
cheers
--
Lorenzo Paulatto - Paris
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