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Hi Riccardo:<div><br><div><div>On Jan 24, 2008, at 12:30 PM, Riccardo Sabatini wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><span class="Apple-style-span" style="-webkit-text-stroke-width: -1; ">diamond structure. For what i understood the CE (cohesive energy) is the </span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">difference between the energy at atoms free and the energy of atoms<span class="Apple-converted-space"> </span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">bounded in the structure you want to study.</div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><br></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><span class="Apple-converted-space"> </span>My first question is about free energy for reference. To evaluate<span class="Apple-converted-space"> </span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">the Si energy alone i made a easy run with one atom in a supercell, big<span class="Apple-converted-space"> </span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">enough to delete all the other atom's effects. The result i had is<span class="Apple-converted-space"> </span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">around -7.47 Ry. My first question is about this result. I'm following a<span class="Apple-converted-space"> </span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">tutorial presentation <span class="Apple-converted-space"> </span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">(<a href="http://mpdc.mae.cornell.edu/Courses/MAE715/Symposium/Veera-Presentation.ppt">http://mpdc.mae.cornell.edu/Courses/MAE715/Symposium/Veera-Presentation.ppt</a>)<span class="Apple-converted-space"> </span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">and there is listed a value of -7.52 Ry for the Si atom free. I tried<span class="Apple-converted-space"> </span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">with different pseudo potentials and with bigger and bigger cells but i<span class="Apple-converted-space"> </span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">can't reproduce that result. I think this difference could arise from a<span class="Apple-converted-space"> </span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">different pseudopotential used by the author and here comes<span class="Apple-converted-space"> </span>my<span class="Apple-converted-space"> </span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">question, is there a reference database for pwscf calculations with the<span class="Apple-converted-space"> </span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">pseudopotentials used by default ? I tried to search but the NIST<span class="Apple-converted-space"> </span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">database i think uses all-electron's calculations (Si atom results to<span class="Apple-converted-space"> </span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">have -288 hartree) and so is not very useful for a fast check...</div></blockquote><div><br class="webkit-block-placeholder"></div>by construction, different pseudopotentials should give equal one-electron levels, but the absolute value of the total energy does depend on the specific pseudopotential. it is only energy differences that are (almost) independent on the specific pseudopotential, as long the PP has been constructed correctly ...</div><div><br><blockquote type="cite"><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; min-height: 14px; "><span class="Apple-style-span" style="-webkit-text-stroke-width: -1; "> Anyway, i did the run for the Si in diamond structure. I tried two </span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">ways, one with a fcc structure with 2 atoms in the cell and another with<span class="Apple-converted-space"> </span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">a sc cell with 8 atoms. The results seem correct but i don't understand<span class="Apple-converted-space"> </span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">what result should i take. </div></blockquote><div><br class="webkit-block-placeholder"></div>either one. the cohesive energy is the total energy of the crystal minus the total energy of an equal number of isolated atoms. of course, for an infinite system this difference is infinite, and you have to decide the refernce with respect to which you want to normalize. if you divide by the total number of atoms, you get a "cohesive anergy per atom" (which I think is the usual definition). for diamond, this is one half the "cohesive energy per cell" (because the unit cell has two atoms). </div><div><br><blockquote type="cite"><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">In the tutorial (the one cited above) there<span class="Apple-converted-space"> </span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">is a value called energy/atom and energy/cell. In the first case i<span class="Apple-converted-space"> </span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">suppose one has to divide the total energy by the numbers of atoms in<span class="Apple-converted-space"> </span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">the cell (2 in the first case and 8 in the second) but i can't figure<span class="Apple-converted-space"> </span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">out what is energy/cell (i tried to divide by the volume but the results<span class="Apple-converted-space"> </span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">are order of magnitude different).</div></blockquote><div><br class="webkit-block-placeholder"></div>total energy per cell = the figure spit out by the code</div><div>total energy per atom = above / (number of atoms in the unit cell)</div><div><br class="webkit-block-placeholder"></div><div>beware, though, that when you perform a calculation for non elementary cells (as the simple cubic cell with 8 atoms that you were mentioning), the result may or may not be directly comparable with the result you get with an elementary cell, according to how careful you have been in the choice of k points for Brillouin-zone sampling. An example may explain this.</div><div><br class="webkit-block-placeholder"></div><div>suppose you perform a calculation with the 8-atom cell and using only the gamma point (k=000) to sample the Brillouin zone. This point would correspond to k=(000) for the elementary cell, but you will easily convince yourself that k=(100) is a reciprocal-lattice vector for the 8-atom cell, while it is not for the 8-atom cell. in conclusion, sampling the 8-atom cell with k=(000) should give an energy which is exactly (within numerical noise) four times as large as the energy of the 2-atom cell, but only if the latter is sampled with the (000) AND the (100) points.</div><div><br><blockquote type="cite"><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><span class="Apple-converted-space"> </span>Is it correct to define a energy for each atom in a cell in this way<span class="Apple-converted-space"> </span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">? (If i remember correctly the difference in energy for the sc<span class="Apple-converted-space"> </span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">simulation and for the fcc arise from the difference in volume, one is 4<span class="Apple-converted-space"> </span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">times than the other not from the numbers of atoms). If yes, i tried to<span class="Apple-converted-space"> </span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">compare the energy/atom for the fcc diamond simulation and for the sc<span class="Apple-converted-space"> </span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">diamond simulation and the results are a little different, can this be<span class="Apple-converted-space"> </span></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; ">caused by the number of plane waves used ?</div></blockquote><div><br class="webkit-block-placeholder"></div>I am not sure that what I said would sounds clear to you, unless you are already a bit familiar with electronic-structure techniques for infinite systems. if you are not, please take a glance, for instance, at the excellent textbook by R. Martin </div><div><br class="webkit-block-placeholder"></div><div><a href="http://www.cambridge.org/uk/catalogue/catalogue.asp?isbn=0521782856">http://www.cambridge.org/uk/catalogue/catalogue.asp?isbn=0521782856</a></div><div><br class="webkit-block-placeholder"></div><div>there used to be a few chapters available for free online, but I cannot find them anymore</div><div><br class="webkit-block-placeholder"></div><div>Stefano B.</div><div><br class="webkit-block-placeholder"></div><div>oh, yes, I was forgetting: please, do not forget to complete your signature with your current affiliation - thanks</div><div> <span class="Apple-style-span" style="border-collapse: separate; border-spacing: 0px 0px; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 14px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: auto; -khtml-text-decorations-in-effect: none; text-indent: 0px; -apple-text-size-adjust: auto; text-transform: none; orphans: 2; white-space: normal; widows: 2; word-spacing: 0px; "><span class="Apple-style-span" style="border-collapse: separate; border-spacing: 0px 0px; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 14px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: auto; -khtml-text-decorations-in-effect: none; text-indent: 0px; -apple-text-size-adjust: auto; text-transform: none; orphans: 2; white-space: normal; widows: 2; word-spacing: 0px; "><span class="Apple-style-span" style="border-collapse: separate; border-spacing: 0px 0px; color: rgb(0, 0, 0); font-family: Helvetica; font-size: 14px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; text-align: auto; -khtml-text-decorations-in-effect: none; text-indent: 0px; -apple-text-size-adjust: auto; text-transform: none; orphans: 2; white-space: normal; widows: 2; word-spacing: 0px; "><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px;"><span class="Apple-style-span" style="font-size: 12px; "><span class="Apple-style-span" style="font-size: 12px; ">---</span></span></span></font></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px;"><span class="Apple-style-span" style="font-size: 12px; "><span class="Apple-style-span" style="font-size: 12px; ">Stefano Baroni - SISSA</span></span></span></font><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px;"><span class="Apple-style-span" style="font-size: 12px; "><span class="Apple-style-span" style="font-size: 12px; "> </span></span></span></font><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px;"><span class="Apple-style-span" style="font-size: 12px; "><span class="Apple-style-span" style="font-size: 12px; ">&</span></span></span></font><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px;"><span class="Apple-style-span" style="font-size: 12px; "><span class="Apple-style-span" style="font-size: 12px; "> </span></span></span></font><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px;"><span class="Apple-style-span" style="font-size: 12px; "><span class="Apple-style-span" style="font-size: 12px; ">DEMOCRITOS National Simulation Center - Trieste</span></span></span></font></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px;"><span class="Apple-style-span" style="font-size: 12px; "><span class="Apple-style-span" style="font-size: 12px; ">[+39] 040 3787 406 (tel) -528 (fax) / stefanobaroni (skype)</span></span></span></font></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; font: normal normal normal 12px/normal Helvetica; min-height: 14px; ; font-size: 12px; "><br style="; font-size: 12px; "></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px;"><span class="Apple-style-span" style="font-size: 12px; "><span class="Apple-style-span" style="font-size: 12px; ">Please, if possible, don't</span></span></span></font><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px;"><span class="Apple-style-span" style="font-size: 12px; "><span class="Apple-style-span" style="font-size: 12px; "> </span></span></span></font><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px;"><span class="Apple-style-span" style="font-size: 12px; "><span class="Apple-style-span" style="font-size: 12px; ">send me MS Word or PowerPoint attachments</span></span></span></font></div><div style="margin-top: 0px; margin-right: 0px; margin-bottom: 0px; margin-left: 0px; "><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px;"><span class="Apple-style-span" style="font-size: 12px; "><span class="Apple-style-span" style="font-size: 12px; ">Why? See:</span></span></span></font><font class="Apple-style-span" size="3"><span class="Apple-style-span" style="font-size: 12px;"><span class="Apple-style-span" style="font-size: 12px; "><span class="Apple-style-span" style="font-size: 12px; "> </span></span></span></font><a href="http://www.gnu.org/philosophy/no-word-attachments.html"><font class="Apple-style-span" color="#0023E9" size="3"><span class="Apple-style-span" style="font-size: 12px;; color: rgb(0, 35, 233); -khtml-text-decorations-in-effect: underline; "><span class="Apple-style-span" style="color: rgb(0, 35, 233); font-size: 12px; -khtml-text-decorations-in-effect: underline; "><span class="Apple-style-span" style="color: rgb(0, 35, 233); font-size: 12px; -khtml-text-decorations-in-effect: underline; ">http://www.gnu.org/philosophy/no-word-attachments.html</span></span></span></font></a></div><div><br class="khtml-block-placeholder"></div><br class="Apple-interchange-newline"></span></span></span> </div><br></div></body></html>