Hi Cameron, I'm a relatively new user of PWscf, but I'll take a try to help you out. I see two potential problems with your setup:
1. I'm not sure that you have the correct ATOMIC_POSITIONS and CELL_PARAMETERS, especially for the ibrav=2 structure type. If you examine the online documentation (http://www.quantum-espresso.org/wp-content/uploads/Doc/INPUT_PW.html), you'll notice that the lattice directions are not the <100>/cubic directions (ibrav=1), but rather <110>-type directions. Thus, I'm not sure that the values you've listed are correct. In particular, it's odd that a few of your CELL_PARAMETERS values are negative. I'm not sure what that's supposed to mean. 2. Secondly, I believe the '2Dxy' specification for cell_dofree is opposite to your intent. I believe you want to specify, and then fix during relaxation, the x-y dimensions of your system and only allow z to change. If that's the case, then the correct option would be cell_dofree=z. Good luck, Jon Zimmerman From: Cameron Foss <cjfoss at umass.edu<mailto:[email protected]>> Reply-To: PWSCF Forum <pw_forum at pwscf.org<mailto:pw_forum at pwscf.org>> Date: Thursday, August 14, 2014 at 2:28 PM To: "pw_forum at pwscf.org<mailto:pw_forum at pwscf.org>" <pw_forum at pwscf.org<mailto:pw_forum at pwscf.org>> Subject: [EXTERNAL] [Pw_forum] Biaxial Strian Silicon Hello all, I am trying to study phonons in silicon under biaxial strain, which occurs when you grow thin films of silicon over a germanium(or SiGe) bulk, which has a slightly larger lattice constant than silicon, thus stretching the silicon out. This type of procedure stretches silicon in the xy directions and slightly compresses it in the z direction. This consequently distorts the symmetry of the crystal. I have ran some strained cases; in which I "strain" the crystal by changing the lattice vectors in the CELL_PARAMETERS CARD and the atomic positions like so: ATOMIC_POSITIONS {alat} Si 0 0 0 Si 0.255 0.255 0.25 CELL_PARAMETERS {alat} -0.51 0.00 0.5 0.00 0.51 0.5 -0.51 0.51 0.00 Whilst the lattice constant remained the equilibrium lattice constant 5.431Angs. However, I would like to use the vc-relax calculation to relax the structure to the strained amount...so I know that the structure is optimized. Although I keep getting an orthogonal operation error. Here is the vc-relax input code and the error: &control calculation='vc-relax' restart_mode='from_scratch', outdir='/home/cameron/QE/espresso-5.1/wktmp', pseudo_dir='/home/cameron/QE/espresso-5.1/pseudo', prefix='relax' / &system ibrav=2, celldm(1)=10.263, nat=2, ntyp=1, ecutwfc =24.0 / &electrons conv_thr = 1.0d-10 mixing_beta = 0.4 / &ions ion_dynamics='damp' / &cell cell_dynamics='damp-w', press=55.0, cell_dofree='2Dxy' / ATOMIC_SPECIES Si 28.0855 Si.pz-vbc.UPF ATOMIC_POSITIONS Si 0.00 0.00 0.00 Si 0.25 0.25 0.25 K_POINTS AUTOMATIC 4 4 4 1 1 1 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% Error in routine checkallsym (2): not orthogonal operation %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% I have two main questions: Did I incorrectly manually "strain" the lattice structure in the first case? And how can I use/fix the vc-relax code to simulate the strained structure? Best, Cameron -------------- next part -------------- An HTML attachment was scrubbed... URL: http://pwscf.org/pipermail/pw_forum/attachments/20140814/71a038e0/attachment.html
