VMD-L Mailing List

From: Ashar Malik (asharjm_at_gmail.com)
Date: Sat Oct 23 2021 - 01:17:03 CDT

Hi,

Unfortunately, I cannot be of any help in this regard.
I have never encountered this problem before and if I did, I would actually
run the MD simulation again.

As I mentioned earlier as well, it is my hunch that the behaviour you are
seeing is because of periodic boundaries not being set up correctly.

I am not familiar with any steps that can fix your problem, but then again
I am not an expert.

Perhaps you could wait here for more input from others who are more
experienced. both to confirm the cause of the problem and if someone might
have seen this behaviour they might suggest a solution.

Sorry for not being able to suggest a fix.

Regards,
Ashar

On Sat, Oct 23, 2021 at 2:01 PM Ryan Woltz <rlwoltz_at_ucdavis.edu> wrote:

> Hi Ashar,
>
> Yes, with your explanation I'm certain the initial boundaries is
> ~145. Sorry for the confusion but the naming is different with files coming
> out of Charmm-gui with the second email being initial config file in step1
> of the setup. Charmm-gui goes through 5 stages of setup. The
> protein +membrane is setup in step3 which is where I enter the "145" number
> and it confirms the protein can fit in this size.
>
> The files that do the initial run of MD is a .inp (input) file and
> I found this line in here.
>
> cellBasisVector1 $a 0.0 0.0; # vector to the next image
> cellBasisVector2 $d $b 0.0;
> cellBasisVector3 0.0 0.0 $c;
> cellOrigin 0.0 0.0 $zcen; # the *center* of the cell
>
> So in the previous response the lines set a,b,c I think this is
> how these variables are being used. From file step5_input.str. These input
> values are again
>
> set a 141.104004
> set b 141.104004
> set c 144.858064
>
> so 141, 141, and 145. apologies for the misunderstanding of terms
>
> If the values are used as boundaries for your entire system (i.e.,
> membrane + water) and the minmax result you provided earlier - then yes.
> Your periodic setup is smaller than your entire system which can explain
> why your "jumps" appear with the system instead of on edges.
>
> How do you recommend we proceed from here? I've been trying to
> wrap things using the protein as my selection and focusing on that but is
> there a way to wrap by size dimensions you calculated? (180,188,158) I know
> this will probably be very time consuming to do it this way, correct?
>
> Thanks for your patience and help,
>
> Ryan
>
> On Fri, Oct 22, 2021 at 10:43 PM Ashar Malik <asharjm_at_gmail.com> wrote:
>
>> Hi Ryan
>>
>>
>>> {"systype":"bilayer","dimensions":["145.078088","145.078088","144.858064","90.0","90.0","90.0"],"input":["namd"],"forcefield":{"type":"c36m","files":["default"],"custom":[]},"hmr":false}
>>>
>>>
>> Again not familiar with the contents of the string above and how each of
>> the values is being used.
>> I am also not familiar with how you set up your system.
>>
>> Taking your system as an example a typical workflow will look
>> something like this
>>
>> A membrane + protein system is constructed which is then minimized
>> followed by solvation/ionization, minimized again and then the system goes
>> into MD.
>>
>> At each step when things are being moved or added your system size can
>> vary.
>>
>> Just before MD, you have to know the system size. The way I am familiar
>> with MD is through the use of a config file which contains line-by-line a
>> collection of variables that you can change the value of. Programs like
>> NAMD will then read that list and the explicit values from the config file
>> will replace system defaults. Some values have no defaults and must be
>> provided.
>>
>> e.g., see here
>> https://www.ks.uiuc.edu/Training/Tutorials/namd/namd-tutorial-unix-html/node26.html
>>
>> Within the description listed in the page above you will notice 4 lines.
>>
>> # Periodic Boundary conditions
>> cellBasisVector1 dx 0. 0. ;# vector to the next image
>> cellBasisVector2 0. dy 0.
>> cellBasisVector3 0. 0 dz
>> cellOrigin 0. 0. 0. ;# the *center* of the cell
>>
>>
>> I have substituted the values to dx, dy, dz for generalization.
>> Your system size before MD (i.e. length along x, y and z axis) needs to be plugged into dx, dy and dz.
>> cell Origin will point to the centre of the box which can be 0, 0, 0 if it's at origin.
>>
>> This now controls the boundaries across which the wrapping will take place. If your system is larger and the boundaries you choose are smaller then I suspect that you will get the behaviour you see. Disclaimer: I have never seen this in my work, but I suspect that the above might be the reason.
>>
>>
>> So I believe my boundaries are "145.078088","145.078088","144.858064" and
>>> compared to your calculations of 180 188 158 these do not match up, correct?
>>>
>>>
>> If the values are used as boundaries for your entire system (i.e.,
>> membrane + water) and the minmax result you provided earlier - then yes.
>> Your periodic setup is smaller than your entire system which can explain
>> why your "jumps" appear with the system instead of on edges.
>>
>>
>>> For clarification, the charmm-gui of 135-145 is the approximate x,y,z
>>> dimensions you give Charmm-gui to create a system for you. So each system I
>>> have (9 of them) I used the input range from 135-145 angs depending on the
>>> system. For this system I'm using as an example I think I used 145 but
>>> there are others that are around 135. Sorry for the confusion, I think this
>>> is irrelevant information and whatever we fix in this system I can apply to
>>> the others.
>>>
>>
>> Does this mean that 145 is your size in x, y and z?
>> Also - are you sure this (145) is the size of your entire system?
>> Membrane + protein + waters?
>>
>>
>>> Apologies for being new to this.
>>>
>>
>> We all start from somewhere.
>>
>> Hopefully the above explanation will clarify things.
>>
>> Regards
>> Ashar
>>
>