[Pw_forum] Intermolecular energy of two fragments.

[Valentina Cantatore]

Thank you very much for all your answers.. I will run calculations again
using your suggestions.

Valentina Cantatore
PostDoc Researcher @ Università del Piemonte Orientale


2014-09-18 14:47 GMT+02:00 Giovanni Cantele <giovanni.cantele at spin.cnr.it>:

> You should carefully check the input geometries and compare them to those
> used in Gaussian.
>
> Using plane waves, what is usually done is to build the supercell with
> A+B, then remove either A or B,
> to calculate E_A and E_B, so that you single out the interaction energy
> (no contribution coming from
> geometry relaxation).
>
> Using your inputs:
> E_A       =     -52.58144317 Ry
> E_A+B   =   -1680.87808274 Ry
> E_B       =   -1628.42761473 Ry
>
> interaction energy: -0.1309 Ry ~ -41 kcal/mol
>
> but the I-Pb-I angle is 126.13 degrees in the A+B system and 103.109 in
> the isolated B system.
> If you use the above prescription (so that the PbI2 fragment has the same
> geometry as in the interacting system),
> you get
> E_B=-1628.29372424
> which corresponds to an interaction energy of ~ +1kcal/mol.
>
> This does not completely solve your issue (e.g. in the former case I find
> a negative interaction energy, whereas in the latter
> a positive one), but should help you in finding the discrepancy with
> respect to the localised basis set calculation.
>
> Giovanni
>
>
> PS within the pseudo potential files you can find suggested values of
> ecutwfc and ecutrho, the ones you use are
> much larger (120Ry/720Ry), maybe rendering the calculation uselessly
> expensive. I used 50Ry/300 Ry, and found
> no variations after increasing those values to 60Ry/360 Ry.
>
>
> On 18 Sep 2014, at 11:44, Valentina Cantatore <valecantatore at gmail.com>
> 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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>
> <MA_20.inp><MAPbI2_20.inp><PbI2_20.inp>
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>
> --
>
> 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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