[DFTB-Plus-User] A MD process including three molecules has failed

[Ben Hourahine]

Dear Zhang,

Other than reducing the time for the MD step (1 fs is potentially too
long for H containing systems, but as with all numerical parameters,
test the effect) and pre-relaxing the structure at the DFTB level, the
input seems OK (you may not want to set the random number seed for a
repeatable choice). I've briefly tried your MD run using a time step of
0.1 fs (this time step still would give energy drifts of a few meV for
NVE simulations in the ps range) and used a DFTB relaxed geometry. I
also find that the cluster dissociates after ~0.25 ps .

It is possible that the DFTB3 model might be more accurate for this case
(damped H related interactions and 3rd order chemical hardness terms,
see the manual for details). Alternatively, might this behaviour be for
a physical reason? I'm not an atmospheric chemist, but would ask
questions like: is this actually a bound system or a transient species
at 200 K? If so, is the thermal transfer rate to the environment and
thermal fluctuations setting the lifetime of this cluster? Should it be
a charged cluster instead of a neutral system?

Regards

Ben

On 22/05/15 11:04, 张海杰 wrote:
>
> Dear Ben:
>
> Thank you for your nice guidance. I have pre-optimised my geometry to a
> good minimum before starting the MD by the M06-2X method. What I am
> describing
> is a common three body collision effect in the atmosphere particle
> formation. So I have 
> set the temperature as 200K to simulate the atmosphere condition. The
> energy at the start
> of the calculation and after dissociation has been show in the
> following picture(unit for Hartree). The
> dissociation happens at about 13ps. The steady state after about 25ps
> stands for sulfuric-ammonia 
> cluster.Here I attached the geom-opt.gen. I am not sure whether I have
> set some wrong parameter? 
> Thank you again for your help.
>
> Best regards,
> Haijie Zhang
>

-- 
      Dr. B. Hourahine, SUPA, Department of Physics,
    University of Strathclyde, John Anderson Building,
            107 Rottenrow, Glasgow G4 0NG, UK.
    +44 141 548 2325, benjamin.hourahine at strath.ac.uk

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