Re: [SIESTA-L] problem in FERMID
Dear all.. Thanks for all your suggestions. I have posted the querry to the smeagol mailing list. Best regards, Asa On Wed, Feb 11, 2009 at 2:25 PM, Stefano Sanvito sanvi...@tcd.ie wrote: Dear Asa, Pablo is right, this sounds more like a Smeagol problem ... in particular if the problem does not happen with Siesta (note however that Smeagol runs on Siesta 1.3). Please re-post your message, with details which will allow us to understand, at: smeagol-disc...@lists.tchpc.tcd.ie Stefano -- = Prof. Stefano Sanvito, Director of Graduate Studies School of Physics School of Physics,Ph. : +353-1-8963065 Trinity College Dublin, Mob.: +353-87-9595384 Dublin 2, IRELAND Fax.: +353-1-6711759 http://www.tcd.ie/Physics/People/Stefano.Sanvito/ = -- Assa Aravindh.S Research Scholar Theory and Computational Studies Section, Material Science Division, IGCAR, India
[SIESTA-L] Fwd: crystal energy calculation
*I am very sorry to re-send my email, but I really need to know the answer.* Dear Siesta users, I am sorry, I have not got my answer reading the whole archive about this subject. Therefore, I hope you would kindly let me know the answer of the following question: We all know that for the crystals we have the following relation: *Cohesive Energy (CE) = (energy of free atom) - (energy of atom in a crystal).* I am working on GaN bulk and nanostructures. 1. Regarding the energy of the free atom, as I read the archive, it seems that we have to consider the ghost atoms surrounding the main atom to get the enrgy of the free atom?! or, it is enough to have the right basis of the atom and calculate its energy? 2. energy of atom in a crystal: when we calculate the enery of the bulk, then how do we have the energy of the atom in the structure? 3. I would be grateful if you let me know the additional terms I have to consider in order to calculate the cohesive energy of a slab too. Regards, Sarah
Re: [SIESTA-L] Fwd: crystal energy calculation
Hello Sarah, Never did that type of calculation, so please sages out there correct me if I'm wrong ... Let say you want to calculate Fe, which is bcc. 1st, perform a calculation with the standard, cubic, two atoms unit cell. 2nd, perform the same calculation (keeping lattice parameter, basis, etc) but putting ghosts at the vertices, namely, keeping only the atom at the center. Finally, compare the two and you'l get the cohesive energy. Best, Roberto On Thu, 12 Feb 2009, Sarah Lebedev wrote: *I am very sorry to re-send my email, but I really need to know the answer.* Dear Siesta users, I am sorry, I have not got my answer reading the whole archive about this subject. Therefore, I hope you would kindly let me know the answer of the following question: We all know that for the crystals we have the following relation: *Cohesive Energy (CE) = (energy of free atom) - (energy of atom in a crystal).* I am working on GaN bulk and nanostructures. 1. Regarding the energy of the free atom, as I read the archive, it seems that we have to consider the ghost atoms surrounding the main atom to get the enrgy of the free atom?! or, it is enough to have the right basis of the atom and calculate its energy? 2. energy of atom in a crystal: when we calculate the enery of the bulk, then how do we have the energy of the atom in the structure? 3. I would be grateful if you let me know the additional terms I have to consider in order to calculate the cohesive energy of a slab too. Regards, Sarah
[SIESTA-L] ForceTol for each atom
For geometry relaxation we will define the MD.MaxForceTol to certain value such as 0.04eV/ang . Anyone know how can I find the force error on each atom for certain step? thank you,
Re: [SIESTA-L] Fwd: crystal energy calculation
1. Regarding the energy of the free atom, as I read the archive, it seems that we have to consider the ghost atoms surrounding the main atom to get the enrgy of the free atom?! or, it is enough to have the right basis of the atom and calculate its energy? It is very desirable to include ghosts, usually they correct the cohesive energy by 0.1-0.3eV. The example as Roberto suggested provides an idea, but in practice it would not work, since in such a case atoms would still be too close. Generally you have to take a much bigger unit cell (~10-15A), place there one atom and the ghosts of all its nearest neighbors as in bulk. Ghost are included by changing the sign of the atom number %block Chemical_Species_Label 1 31 Ga 1 -7 N#ghost %endblock Chemical_Species_Label You still have to provide the same basis and pseudopotential file that you used for bulk calculations. 2. energy of atom in a crystal: when we calculate the energy of the bulk, then how do we have the energy of the atom in the structure? I don't understand this question. 3. I would be grateful if you let me know the additional terms I have to consider in order to calculate the cohesive energy of a slab too. You mean surface energy? For binary compounds it's indeed a problem. You have the cohesive energy for a pair not for a single atom. But you can find the upper and lower bounds of the chemical potential of each atom assuming that each atom might come from the respective bulk (one bound) or from a gas phase (another bound). Then people normally plot Energy vs. chemical potential and see which of the surface reconstructions is the most stable depending on the growth conditions. I think the best case for you would be to learn about GaAs surface reconstructions which is quite close to your system and was investigated very intensely. Check for example: GaAs http://prola.aps.org/abstract/PRB/v54/i12/p8844_1 http://prola.aps.org/abstract/PRB/v62/i12/p8087_1 GaN http://prola.aps.org/abstract/PRB/v65/i24/e245212 http://prola.aps.org/abstract/PRL/v88/i6/e066103
