[CP2K:1637] Energy dependence on the position of the system in the box

Laino Teodoro teodor... at gmail.com
Sat Dec 13 12:46:40 UTC 2008

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.


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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