[Pw_forum] Intermolecular energy of two fragments.

Giuseppe Mattioli giuseppe.mattioli at ism.cnr.it
Thu Sep 18 11:13:51 CEST 2014

Dear Valentina
I suppose you have done everything you must do to subtract the basis  
set superposition error (BSSE) in Gaussian... I also suppose that you  
know that Gaussian uses Hartree atomic units, while QE uses Rydberg a.u.
Moreover, you should discover a little more of your work, if you want  
some meaningful help. Your procedure is sound in a  
plane-wave/pseudopotential framework (no BSSE...), but are you sure  
you're using the very same density functional setup in Gaussian and  
QE? What about dispersion forces? Which "assume_isolated" option? The  
'mt' trick usually requires a supercell quite larger than the isolated  
system placed therein. You should check the convergence of your  
calculations wrt to such parameter.

Giuseppe Mattioli

Quoting Valentina Cantatore <valecantatore at gmail.com>:

> 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

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