[ADF-LIST] Non-physically large charges produced by EEM

Alexei Yakovlev yakovlev at scm.com
Tue Oct 3 08:56:00 CEST 2017

Dear Gayani,

The following paragraph from 
https://www.scm.com/doc/ReaxFF/Error_messages.html explains a bit what 
happens and how you can ignore the message:

    For every atom type, the eta and gamma parameters for the
    electronegativity equalization method (EEM) should satisfy the
    relation: eta > 7.2*gamma. Otherwise a polarisation catastrophe is
    likely to occur at a short inteatomic distance, the critical
    distance being dependent on the eta/gamma ratio. The smaller the
    ratio the larger the distance at which this occurs. When the
    polarisation catastrophe occurs the amount of charge that flows from
    one atom to the other can become very large. The EEM routine checks
    that the resulting charge for each atom lies within the [-10,Z]
    interval, where Z is the number of electrons in the neutral atom,
    and throws an error if the check fails. The user can override this
    error by setting the badeem control parameter to 1.

If the atom that fails to satisfy the q∊∊[-10,Z] condition is hydrogen 
that gets a charge slightly over +1 then you might be able to avoid this 
error by further reducing the time step (and thus reducing the 
integration error).

Kind regards,

On 03/10/2017 03:10, Gayani Nadeera Pallewela (Dr) wrote:
> Dear All,
> I’m performing reaxff MD simulations on energetic materials. (RDX and 
> HMX).
> Temperature =3000K
> Pressure about 35399.33M Pa
> This is a compressed system that have density about 2.53 g/cm3
> NVT simulation. Time step is 0.1 fs.
> HE.ff :force field
> After about 28000 iterations the simulation was terminated by giving 
> an error message : Non-physically large charges produced by EEM
> I got this error when I do high density high temp systems.
> I searched for this error and  found may be I have to change the 
> initial structure or the force field.
> Several authors have used HE.ff for these type of simulations.
> Can you please comment on this problem
> Thanks.
> Best regards,
> Gayani
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