From: Aron Broom (broomsday_at_gmail.com)
Date: Sun Apr 20 2014 - 00:27:37 CDT
Actually, in thinking about that a bit more, you'll probably need to
include some kind of upper bound on those colvars.
When I've used this kind of technique before, the functional form of the
potential was always Gaussian and not harmonic. In the harmonic case the
potential doesn't flatten out, so things will get crazy as your ions get
further and further away. But if you put an upper bound on the colvar that
is as close as you'll allow an ion to get (but don't put any wall
constants, because you actually want the thing to go past that wall) then I
think what will happen is the harmonic potential will accelerate your ions
away from the protein, until they pass that upper bound, at which point it
will stop affecting them, which is exactly what you want.
I'm guessing a bit about how exactly the colvar module treats things that
are beyond the boundary.
On Fri, Apr 18, 2014 at 12:15 AM, Aron Broom <broomsday_at_gmail.com> wrote:
> So this ought to be possible:
>
> If you use either a colvar like this (but without the boundaries) or just
> use a simple distance colvar to the COM of your protein, then make a
> harmonic using that, but with a slightly abnormal force constant.
>
> The key would be, you make the force constant for the harmonic negative.
> This should create, rather than a well centered at the protein, a
> repulsive hill centered at the protein, and then you can control how far
> from the protein the ions tend to get by adjusting the colvar width. This
> is essentially how one does the "keep away" rather than "come towards" you
> mentioned.
>
> I've used this technique in other MD programs, but never tested it in
> NAMD. I don't see why it wouldn't work though.
>
> ~Aron
>
>
> On Thu, Apr 17, 2014 at 7:36 PM, Chris Ing <ing.chris_at_gmail.com> wrote:
>
>> I'd like to prevent ions from interacting with my membrane-embedded
>> protein using collective variables. I wanted to avoid using a modification
>> of keep_waters_out.tcl because it seemed too complex / slow. To make
>> matters simpler, I only have a handful of neutralizing ions in the box so I
>> can make a separate variable for each one.
>>
>> I've tried this colvar code below without success. Ions on one side of
>> the bilayer seem to be restrained quite nicely but those on the opposite
>> side of the box are flocking like crazy to the lipid interface trying to
>> get through the bilayer (to what I imagine is the +20-40 Ang region).
>>
>> colvar {
>> name restrain01
>> width 1.0
>> lowerBoundary 20.0
>> upperBoundary 40.0
>> lowerwallconstant 0.5
>> upperwallconstant 0.5
>> distanceZ {
>> # Ion #1
>> main {
>> atomNumbers { 60984 }
>> }
>> # C.O.M. of Protein
>> ref {
>> atomNumbers { 1244 5633 2707 4170 }
>> }
>> axis ( 0.0, 0.0, 1.0 )
>> }
>> }
>>
>> If I use the distanceZ "period 1.0" flag, I get the warning:
>> Lower wall and upper wall are equal in the periodic domain of the colvar:
>> disabling walls.
>> and the ions don't go anywhere.
>>
>> I need to use a specific version of NAMD 2.9 for this task.
>>
>> It seems like this could be a common problem, but I was unable to find
>> anything on namd-l about restraining things "away from a target" as opposed
>> to restraining to a target used in umbrella sampling.
>>
>> Thanks,
>> -Chris Ing
>>
>
>
>
> --
> Aron Broom M.Sc
> PhD Student
> Department of Chemistry
> University of Waterloo
>
-- Aron Broom M.Sc PhD Student Department of Chemistry University of Waterloo
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