[Pw_forum] Intermolecular energy of two fragments.

[Paolo Giannozzi]

The safe way to make this kind of comparisons is to compute the energy
of the separate units starting from the same data of the complete
system without one of the two units, and not touching anything else.
Your atomic positions are not the same in the input files you sent.

Paolo

On Thu, 2014-09-18 at 11:44 +0200, Valentina Cantatore wrote:
> 1) Yes
> 2) Yes
> 3) Yes 
> 
> 
> I attach to the mail my three inputs. Thank you, again, for your help.
> 
> 
> Valentina Cantatore
> PostDoc Researcher @ Università del Piemonte Orientale 
> 
> 2014-09-18 11:26 GMT+02:00 Giovanni Cantele
> <giovanni.cantele at spin.cnr.it>:
>         
>         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
>         
>         
>         
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-- 
 Paolo Giannozzi, Dept. Chemistry&Physics&Environment, 
 Univ. Udine, via delle Scienze 208, 33100 Udine, Italy
 Phone +39-0432-558216, fax +39-0432-558222