Hi,
I tried to keep the geometry of the BF4 fixed by using constraints using
lincs. But , unfortunately, my simulation is crashing immediately and if I try
minimization with only 2 molecules, it provides a lot of LINCS warning and
generate a lot of step*.pdb file . If I try to visualize the minimized snapshot
in VMD, it looks like all the distances I tried to constrain decreased
drastically. Finally, trying MD run with this "minimized" configuration results
in crashing due to bad contacts.
I am sure I am doing something wrong and it might be that my itp file is wrong
. So any help will be highly appreciated.
Here is the details of what I did.
The geometry of the molecule is tetrahedral with B at the center and 4 F
atoms is surrounding it in a tetrahedral manner.
I first generated a itp file for BF4 which is shown below: I first got the LJ
parameters and charges for B and F atom and put them in ffoplsnb.itp file as
new
atom types opls_1014 and opls_1015 . Initially I tried to put contsraint along
all bonds ( i.e among F atoms as well ). But, grompp provides warning that
number of constraint is more than number of degrees of freedom. So, I reduced
number of constraints by only putting constraint among B and F. But, it did not
work either.
Here is the .itp file I wrote for rigid BF4 . It will be great if someone can
point me what I am doing wrong.
[ moleculetype ]
; molname nrexcl
BF4 3
[ atoms ]
#ifdef _FF_OPLS
1 opls_1014 1 BF4 B 1 0.8276
2 opls_1015 1 BF4 F1 1 -0.4569
3 opls_1015 1 BF4 F2 1 -0.4569
4 opls_1015 1 BF4 F3 1 -0.4569
5 opls_1015 1 BF4 F4 1 -0.4569
#endif
[ constraints ]
1 2 1 0.146
1 3 1 0.146
1 4 1 0.146
1 5 1 0.146
; 2 3 1 0.238
; 2 4 1 0.238
; 2 5 1 0.238
; 3 4 1 0.238
; 3 5 1 0.238
; 4 5 1 0.238
[ exclusions ]
1 2 3 4 5
2 1 3 4 5
3 1 2 4 5
4 1 2 3 5
5 1 2 3 4
________________________________
From: Mark Abraham <mark.abra...@anu.edu.au>
To: Discussion list for GROMACS users <gmx-users@gromacs.org>
Sent: Wed, April 27, 2011 8:39:23 PM
Subject: Re: [gmx-users] rigid tetrahedral molecule
On 4/28/2011 11:25 AM, Justin A. Lemkul wrote:
>
>
> Sanku M wrote:
>> Hi,
>> I am interested in simulating a anionic molecule BF4(-) ( Boron
>>tetrafluoride). In the paper which developed the parameters for this
>>molecule,
>>it is mentioned that it has been used as 'rigid' molecule i.e the molecule
>>only
>>has non-bonding interaction but there was no intramolecular motion as the
>>geometry was fixed.
>> I am trying to simulate this molecule in gromacs treating it as rigid.
>> But, I
>>was looking for best way to 'rigidify' this molecule.
>>
>> I was wondering whether using LINCS to constrain all B-F and F-F bonds will
>> be
>>good enough . Or, Should I use virtual sites ? If I really need to use
>>virtual
>>site, will it be something like TIP5P water model ?
>> Can someone suggest the best wayout ?
>>
>
> Constraints should do the trick, but probably the best approach is to simply
>contact the authors who developed the model and ask how they did it. Then you
>know you're exactly reproducing what they did.
Yep.
Be aware that the coupled constraints make life tricky, and you should read up
in the manual and literature for how best to use P-LINCS in such cases.
Algorithms like SETTLE for rigid water exist for a reason...
Mark
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