Okay, same thing with 0.5V/nm. I think it's fairly safe to say that there's something wrong here...
Alex On Wed, Nov 8, 2017 at 12:25 PM, Alex <nedoma...@gmail.com> wrote: > Good question. Dielectric breakdown of water is generally poorly > understood and the threshold depends on the ionic strength, but 0.4-0.5V/nm > is generally where the fun begins. MD modelers working with solvated > systems casually ignore this, unless they have the great misfortune of > getting me as a reviewer. :) > That aside, I believe your suggestion is sound, at least to see if what I > observe is an outright bug. > > Thanks, > > Alex > > On Wed, Nov 8, 2017 at 10:39 AM, Dan Gil <dan.gil9...@gmail.com> wrote: > >> Yes I saw your plot and it is simply around 0 with walls. >> >> What is the field required for dielectric breakdown? >> >> On Wed, Nov 8, 2017 at 12:18 PM, Alex <nedoma...@gmail.com> wrote: >> >> > Hi Dan, >> > >> > Yup, periodic, continuous, and electrically neutral. I suggested a >> similar >> > thought in my question, i.e. with walls any transport would definitely >> be >> > transient and self-limited. However, nothing is transported even in the >> > perturbative sense, as you can see from the flux. The behavior is that >> of a >> > system without any driving field. >> > >> > The electric field is already quite high (0.1 V/nm) and of course I >> could >> > go completely nuts and exceed the experimental dielectric breakdown >> > threshold values for water, but the question remains, no? >> > >> > Thanks, >> > >> > Alex >> > >> > >> > >> > On 11/8/2017 9:58 AM, Dan Gil wrote: >> > >> >> Hi Alex, >> >> >> >> Is your system without walls periodic and continuous in all >> directions? I >> >> can see a scenario where this sort of system will maintain charge >> >> neutrality in the different reservoirs separated by the semi-porous >> >> membrane. While cations will be transported, the charge in each >> reservoir >> >> will be maintained constant because as one cation leaves, its periodic >> >> image enters the same reservoir. It is a steady-state process. >> >> >> >> In the system with walls, charge neutrality will be broken if cations >> are >> >> transported across the membrane because it won't have a periodic image >> >> that >> >> enters the same reservoir as it leaves. I think that the cation >> transport >> >> would be more like capacitance since a constant electric field will >> only >> >> be >> >> able to hold a finite number of cations across the membrane. This is an >> >> equilibrium process. >> >> >> >> Maybe try higher electric field? >> >> >> >> Dan >> >> >> >> On Fri, Nov 3, 2017 at 2:43 AM, Alex <nedoma...@gmail.com> wrote: >> >> >> >> Hi all, >> >>> >> >>> It appears that the external field is refusing to move the ions when >> >>> walls >> >>> are present. I am comparing two setups of a system that has an aqueous >> >>> bath >> >>> (1M KCl) split by a semi-porous (infinitely selective for cations) >> >>> membrane >> >>> in XY. The only difference between them is that one is periodic in XYZ >> >>> and >> >>> the other has two walls. The difference isn't minor -- consider K+ >> fluxes >> >>> with and without walls: https://www.dropbox.com/s/jve0 >> >>> hqqpfkn4ui6/flux.jpg?dl=0 >> >>> >> >>> Initially, ionic populations in each case are homogeneous. I realize >> that >> >>> with walls the process will stop when all cations end up at the top of >> >>> the >> >>> box (and that's the goal). However, there is no flux right from the >> >>> start. >> >>> Relevant portion of the mdp with walls below (not sure if this is >> >>> important, but 'ewald-geometry' directive isn't in the mdp without >> >>> walls): >> >>> >> >>> pbc = xy >> >>> nwall = 2 >> >>> wall-type = 12-6 >> >>> wall-r-linpot = 0.25 >> >>> wall_atomtype = opls_996 opls_996 >> >>> wall-ewald-zfac = 3 >> >>> periodic_molecules = yes >> >>> ns_type = grid >> >>> rlist = 1.0 >> >>> coulombtype = pme >> >>> ewald-geometry = 3dc >> >>> fourierspacing = 0.135 >> >>> rcoulomb = 1.0 >> >>> rvdw = 1.0 >> >>> vdwtype = cut-off >> >>> cutoff-scheme = Verlet >> >>> >> >>> Any ideas? >> >>> >> >>> Thanks, >> >>> >> >>> Alex >> >>> >> >>> -- >> >>> Gromacs Users mailing list >> >>> >> >>> * Please search the archive at http://www.gromacs.org/Support >> >>> /Mailing_Lists/GMX-Users_List before posting! >> >>> >> >>> * Can't post? 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