[gmx-users] Shake constraint problem

[K.A. Feenstra]

Attila Borics wrote:
> 
> Hi!
> 
> I try simulate a 4-residue peptide with oplsaa, using tip4p, tip3p and
> spc water model.

First a note: you probably shouldn't use spc water with opls, since those
are different force fields. They don't usually mix well...

> In all cases my postion-restraint dynamics stops with the following
> error message: (prlog0.log)
> 
> Constraint error in algorithm Shake at step 2449
> Fatal error: ci = -2147483648 should be in 0 .. 63 [FILE nsgrid.c, LINE
> 210]

Possibly you are constraining to a different set of coordinates that
that from which you are starting. This might cause a molecule to be
torn apart in such a way that Shake cannot compensate.

> Here's my *.mdp. Everything was working fine for this system when using
> gromos96. Do I need to change something, use settle, etc.
> If I need to use settle, how can I do that?

Your parameters look fine to me. 
Only one remark: you have unconstrained_start = yes,vwhich is not a good 
idea in general; this is only advised for continuation runs.

In the file spc.itp 'settles' are defined in stead of bonds/constraints. 
So that is the default. If you want bonds in stead, use flexspc.


-- 
Groetjes,

Anton
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|        | Anton Feenstra                                            |
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