Energy dependence on the position of the system in the box

ilya ily... at
Mon Dec 15 12:01:24 UTC 2008

Thanks Teo, i'll try this.

On Dec 13, 3:46 pm, Laino Teodoro <teodor... at> wrote:
> There could be two reasons:
> 1) do you get the same behavior for QM only (same setup, cell,  
> cutoff, XC, MT)?
> If yes then:
> That's an intrinsic behavior of plane waves codes. QS uses PW for the  
> electrostatics and XC.
> Electrostatics converge quite quickly w.r.t. cutoff.
> XC instead not. Try using a larger cutoff. In the limit of an  
> infinite cutoff this effect will disappear.
> 2) If this effect is not present for pure QM calculations then the  
> reason is that your grid levels
> are not good enough to map properly the electrostatic potential with  
> the GEEP scheme.
> In this case you'll have to play properly with number of grid levels  
> and cutoffs.
> It may take a while to find out the proper setup.
> Teo
> On 13 Dec 2008, at 12:18, ilya wrote:
> > Hello!
> > I am running QM/MM calculation of nonperiodic system. I noticed that
> > the energy of the system is highly dependent on the position of the
> > system in the simulation box. For example even small translations can
> > change the energy for 10 kcal/mol. The system has a charged qm and MM
> > parts. I am using MT poisson solver for MM and QM electrostatics. The
> > box (both for QM and MM) is approximately twice as large as the system
> > as required for MT solver. The dependence is strange because MT should
> > be translational invariant.
> > I need to find the energy of reag, ts and products of the reaction.
> > During the optimization the geometry of the system changes and the
> > resulting relative energies depend on weather i center the system
> > every step in the QM box or not for example.
> > Could someone give me a hint how to make the simulation correctly. Is
> > such dependence a normal behavior?
> > Best regards.
> > Ilya.

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