[gmx-users] Ewald summation in 2 dimensions with correction

[David van der Spoel]

On 11/11/17 16:44, Dan Gil wrote:
> Hi,
> 
> Just wanted to see if anyone had any thoughts...
> 
> For more information, if I use:
> wall-ewald-zfac = 3
> E-z                    = 1 5 0
> The electric field from ionic liquid has a magnitude of 7.5 V/nm in the
> opposite direction to the applied electric field. This makes no sense to me
> at all.
> 
> With
> wall-ewald-zfac = 5
> E-z                    = 1 5 0
> The ionic liquid E is -6.2
How do you measure the electric field? Using gmx potential?
What numbers does the (optional?) file field.xvg contain?

There can of course be a difference between applied field and effective 
field, in particular with PBC. I sugges you read the supporting 
information for our paper: Carl Caleman and David van der Spoel: 
Picosecond Melting of Ice by an Infrared Laser Pulse - A Simulation 
Study Angew. Chem. Intl. Ed. 47 pp. 1417-1420 (2008)
> 
> With
> wall-ewald-zfac = 10
> E-z                    = 1 5 0
> The electric field is still slightly above the expected 5 V/nm.
> 
> The dimensions of the box is about 5x5x20 nm3.
> 
> Reading various sources say that the wall factor should not matter as long
> as it is sufficiently large (x3) but in my case it seems to matter a lot.
> Should I report this as a bug or do you think I can investigate this
> further?
> 
> Best Regards,
> 
> Dan
> 
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> 
> On Wed, Nov 8, 2017 at 11:37 AM, Dan Gil <dan.gil9973 at gmail.com> wrote:
> 
>> Hello,
>>
>> I am studying room-temperature ionic liquids in slab geometry and I am
>> experiencing that I cannot get rid of the interactions between periodic
>> images in the z-direction. Is there something wrong with what I am doing or
>> is this inevitable?
>>
>> The reason I think this is because when I change "wall-ewald-zfac" the
>> results change also.
>>
>> Gromacs 5.1
>> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
>> ;Integration Method and Parameters
>> integrator               = md
>> nsteps                   = 4000000
>> dt            = 0.002
>> nstenergy                = 100
>> nstcalcenergy            = 100
>> nstlog                   = 5000
>>
>> ;Output Control
>> nstxout = 100
>> nstvout = 10000000
>>
>> ;Cutoff Schemes
>> cutoff-scheme            = verlet
>> rlist                    = 1.0
>> vdw-type                 = cut-off
>> rvdw                     = 1.0
>> DispCorr                 = EnerPres
>>
>> ;Coulomb interactions
>> coulombtype              = pme
>> rcoulomb                 = 1.0
>> fourierspacing           = 0.4
>>
>> ;Constraints
>> constraints              = all-bonds
>>
>> ;Temperature coupling
>> gen-vel                  = yes
>> tcoupl                   = v-rescale
>> tc-grps                  = System
>> tau-t                    = 0
>> ref-t                    = 300
>>
>> ;Walls
>> pbc                      = xy
>> nwall                    = 2
>> wall-type                = 10-4
>> ewald-geometry           = 3dc
>> wall-atomtype            = cp cm
>> wall-density             = 5 5
>> wall-ewald-zfac          = 10
>>
>> ;Electric Field
>> ;Cosine with freq=0, Field V/nm, Phase=n/a
>> E-z                      = 1 0 0
>> - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
>>
>> Best Regards,
>>
>> Dan
>>


-- 
David van der Spoel, Ph.D., Professor of Biology
Head of Department, Cell & Molecular Biology, Uppsala University.
Box 596, SE-75124 Uppsala, Sweden. Phone: +46184714205.
http://www.icm.uu.se