[gmx-users] Three-site model for acetonitrile

[Christoph Freudenberger]

Christoph Freudenberger wrote:
> I do not use any constraints. While double-checking the forcefield
> of MeCN i realized, I had set the bond length of the C-N triple bond
> to small (0.1nm instead of 0.116nm).
> 
> Could that be the reason?
> I just started a simulation to find out...
The simulation is running now, and i looks like it really makes some
difference... But I have to wait for the longterm behaviour of the
system to be sure.

Just to get this thing complete:
What would be a good guess for the bond and angle force constants?
For now my acetonitrile.itp looks like this:
[ bonds ]
;  ai  aj funct           c0           c1
1       2       1          0.14600     334720. ;C-C
2       3       1          0.11600     502080. ;C#N

[ angles ]
;  ai    aj    ak       funct   c0      c1
     1     2     3       1       180.0  418.400 ;C-C#N

The values were guessed by using the prodrg server. (BTW: Does anyone know
why the force constants for bonds given by prodrg are always factor 10
larger than the corresponding values in ffgmxbon.itp? Anyway, the values
I chose are consistent with ffgmxbon.itp.)

The values for the C-C single bond and the angle should be ok in my option,
but maybe the one for the C-N triple bond should be larger? The value
is equal to a C=O carbonyl bond.

regards
-- 
Christoph Freudenberger
Abt. Organische Chemie I AK Siehl - Uni Ulm -Tel: ++49-731-502-2785