[Pw_forum] Intermolecular energy of two fragments.

[Giovanni Cantele]

On 18 Sep 2014, at 10:40, Valentina Cantatore <valecantatore at gmail.com> wrote:

> Good morning to everyone.
> 
> I would like to calculate the interaction energy between to molecules using QE. What I set up was to fix the dimension of a cubic box at 20 A (I think it is big enough to avoid interactions between the other replicated molecules), calculate the energy of each of the two fragments (Ea and Eb) and then calculate the energy of the two fragments together (Eab). What I expect is that interaction energy (Ein) is given by Ein=Eab-(Ea+Eb).
> 
> I also used the assume_isolated option for all calculations. I sampled only gamma points.
> 
> The results obtained are not of the same order of magnitude of those obtained with a gaussian basis set on the same systems.
> 
> Where am I wrong? Is it possible run this kind of calculation with Quantum Espresso? Thank you
> 
> Valentina Cantatore
> PostDoc Researcher @ Università del Piemonte Orientale 
> 


the procedure you describe is correct and this kind of calculations is of course possible with QE. Just few questions:
1) are you using the same XC approximation with gaussian and plane waves runs (e.g. PBE)?
2) are you sure that both calculations are converged with respect to the respective parameters (in particular basis set size)?
3) did you carefully check the energy units used by the two different codes?

Otherwise, you should provide more details and/or inputs to understand what is going on.

Giovanni

-- 

Giovanni Cantele, PhD
CNR-SPIN
c/o Dipartimento di Fisica
Universita' di Napoli "Federico II"
Complesso Universitario M. S. Angelo - Ed. 6
Via Cintia, I-80126, Napoli, Italy
e-mail: giovanni.cantele at spin.cnr.it
Phone: +39 081 676910
Skype contact: giocan74

ResearcherID: http://www.researcherid.com/rid/A-1951-2009
Web page: http://people.na.infn.it/~cantele