-------- Forwarded Message -------- From: Herman van Midden <[email protected]> To: mailman-owner at democritos.it Subject: Calculating modulated structures wit PWscf Date: Thu, 19 Dec 2013 15:09:16 +0100
Hi I am rather new in the field of PWscf calculations and I am hoping someone can help me to answer some, maybe obvious questions. I would like to perform total energy calculations on structures modulated with a change density wave (CDW). These CDW's usually extend over several unit cells. So my approach is to first calculate the equilibrium structure for one unit cell, repeat these unit cell in the desired direction and modulate it with the CDW. If my model gives a good description of the material, the total energy of the modulated structure should be lower than that of the unmodulated structure. The problem of this approach is that the repeated unmodulated structure contains a large number of atoms. How can I calculate the total energy of this unmodulated structure? I expected I would be able to use the total energy of one unit cell, calculated with N times more k-points in the direction where I am going to extend the unit cell N-times. Repeating the unit cell N times in real space, should be equivalent to reducing the k point sampling by a factor N in reciprocal space. Or Not? This approach worked for some potentials, but now I am running into problems. I am currently using the GBRV potentials which use ecutwfc=40 and ectrho=200 Ry. Using this approach results in a total energy which is larger than N times the unit cell energy, and subsequent vc-relax steps decrease the energy. So it is not the desired unmodulated equilibrium structure. The calculations also take much longer than I expected. I hoped that simultaneously increasing the number of atoms and reducing the number of k-points would result in a comparable computer load. When I compare the outputs I noticed that the fft mesh increases with the number of atoms. Can I reduce n1,n2 or n3 to optimize calculation speed? Or will this significantly reduce the reliability of the calculation? Does anyone have any suggestions how I can perform total energy calculations on CDW modulated structures. Any help is welcome Herman van Midden Institute Jozef Stefan Ljubljana Slovenia -- Paolo Giannozzi, Dept. Chemistry&Physics&Environment, Univ. Udine, via delle Scienze 208, 33100 Udine, Italy Phone +39-0432-558216, fax +39-0432-558222
