[gmx-users] vacuum right but melt collapse

[Mark Abraham]

kitty ji wrote:
> Hi GMX user:
> 
> A model for Poly(vinyl methyl ether) was built with OPLS force field ( 
> all H ).
> 
>  
> 
> When the model was run alone (a single chain) in vacuum, anything goes ok.
> 
>  
> 
> But with 45 chains box, the system will be collapse with a little bond 
> elongates abnormal whatever in NVT or NPT.
> 
>  
> 
> The relax time of pressure and temperature were adjust but no helpful.
> 
>  
> 
> How can I found some possible reason and then fix it ? Any advise will 
> be appreciated.

If an isolated molecule doesn't do anything strange, and two more more
do, then you have a problem with inter-molecular interactions, i.e. the
non-bonded ones. Try with just 2 chains to verify this... and look
carefully at the atom types and the non-bonded parameters for them.

Mark