<html><head><meta http-equiv="Content-Type" content="text/html charset=iso-8859-1"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;"><div>You should carefully check the input geometries and compare them to those used in Gaussian.</div><div><br></div><div>Using plane waves, what is usually done is to build the supercell with A+B, then remove either A or B,</div><div>to calculate E_A and E_B, so that you single out the interaction energy (no contribution coming from</div><div>geometry relaxation).</div><div><br></div><div>Using your inputs:</div><div>E_A = -52.58144317 Ry<br>E_A+B = -1680.87808274 Ry<br>E_B = -1628.42761473 Ry</div><div><br></div><div>interaction energy: -0.1309 Ry ~ -41 kcal/mol</div><div><br></div><div>but the I-Pb-I angle is 126.13 degrees in the A+B system and 103.109 in the isolated B system.</div><div>If you use the above prescription (so that the PbI2 fragment has the same geometry as in the interacting system),</div><div>you get</div><div>E_B=-1628.29372424</div><div>which corresponds to an interaction energy of ~ +1kcal/mol.</div><div><br></div><div>This does not completely solve your issue (e.g. in the former case I find a negative interaction energy, whereas in the latter</div><div>a positive one), but should help you in finding the discrepancy with respect to the localised basis set calculation.</div><div><br></div><div>Giovanni</div><div><br></div><div><br></div><div>PS within the pseudo potential files you can find suggested values of ecutwfc and ecutrho, the ones you use are</div><div>much larger (120Ry/720Ry), maybe rendering the calculation uselessly expensive. I used 50Ry/300 Ry, and found</div><div>no variations after increasing those values to 60Ry/360 Ry.</div><div><br></div><br><div><div>On 18 Sep 2014, at 11:44, Valentina Cantatore <<a href="mailto:valecantatore@gmail.com">valecantatore@gmail.com</a>> wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><div dir="ltr">1) Yes<div>2) Yes</div><div>3) Yes </div><div><br></div><div>I attach to the mail my three inputs. Thank you, again, for your help.</div><div><br></div><div><div style="font-family:arial,sans-serif;font-size:13px;color:rgb(136,136,136)">Valentina Cantatore</div><div style="font-family:arial,sans-serif;font-size:13px;color:rgb(136,136,136)">PostDoc Researcher @ Università del Piemonte Orientale </div></div></div><div class="gmail_extra"><br><div class="gmail_quote">2014-09-18 11:26 GMT+02:00 Giovanni Cantele <span dir="ltr"><<a href="mailto:giovanni.cantele@spin.cnr.it" target="_blank">giovanni.cantele@spin.cnr.it</a>></span>:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div class="HOEnZb"><div class="h5"><br>
On 18 Sep 2014, at 10:40, Valentina Cantatore <<a href="mailto:valecantatore@gmail.com">valecantatore@gmail.com</a>> wrote:<br>
<br>
> Good morning to everyone.<br>
><br>
> 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).<br>
><br>
> I also used the assume_isolated option for all calculations. I sampled only gamma points.<br>
><br>
> The results obtained are not of the same order of magnitude of those obtained with a gaussian basis set on the same systems.<br>
><br>
> Where am I wrong? Is it possible run this kind of calculation with Quantum Espresso? Thank you<br>
><br>
> Valentina Cantatore<br>
> PostDoc Researcher @ Università del Piemonte Orientale<br>
><br>
<br>
<br>
</div></div>the procedure you describe is correct and this kind of calculations is of course possible with QE. Just few questions:<br>
1) are you using the same XC approximation with gaussian and plane waves runs (e.g. PBE)?<br>
2) are you sure that both calculations are converged with respect to the respective parameters (in particular basis set size)?<br>
3) did you carefully check the energy units used by the two different codes?<br>
<br>
Otherwise, you should provide more details and/or inputs to understand what is going on.<br>
<br>
Giovanni<br>
<br>
--<br>
<br>
Giovanni Cantele, PhD<br>
CNR-SPIN<br>
c/o Dipartimento di Fisica<br>
Universita' di Napoli "Federico II"<br>
Complesso Universitario M. S. Angelo - Ed. 6<br>
Via Cintia, I-80126, Napoli, Italy<br>
e-mail: <a href="mailto:giovanni.cantele@spin.cnr.it">giovanni.cantele@spin.cnr.it</a><br>
Phone: <a href="tel:%2B39%20081%20676910" value="+39081676910">+39 081 676910</a><br>
Skype contact: giocan74<br>
<br>
ResearcherID: <a href="http://www.researcherid.com/rid/A-1951-2009" target="_blank">http://www.researcherid.com/rid/A-1951-2009</a><br>
Web page: <a href="http://people.na.infn.it/~cantele" target="_blank">http://people.na.infn.it/~cantele</a><br>
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-- <br><br>Giovanni Cantele, PhD<br>CNR-SPIN<br>c/o Dipartimento di Fisica<br>Universita' di Napoli "Federico II"<br>Complesso Universitario M. S. Angelo - Ed. 6<br>Via Cintia, I-80126, Napoli, Italy<br>e-mail: <a href="mailto:giovanni.cantele@spin.cnr.it">giovanni.cantele@spin.cnr.it</a><br>Phone: +39 081 676910<br>Skype contact: giocan74<br><br>ResearcherID: <a href="http://www.researcherid.com/rid/A-1951-2009">http://www.researcherid.com/rid/A-1951-2009</a><br>Web page: <a href="http://people.na.infn.it/~cantele">http://people.na.infn.it/~cantele</a><br>
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