Dear Furman: I think, the easier and better way to make such a "compress" is using isostatic pressure, which can be specified by parameter "press" in "cell" namelist, especially for the un-cubic system. For example, in HCP structure, the cell is determined by "a" and "a/c", while the volume changing, not only "a", but also "a/c" ratio changes, too. Therefore, you can do a batch of "vc-relax" tasks with different press and gather the final volume and total energy to fit EOS. The aim compress, as 10% or 20% shrinkage, can be found from EOS fitting result, where you can obtain the P-V curve.
-- GAO Zhe CMC Lab, Materials Science & Engineering Department, Seoul National University, South Korea At 2012-06-08 15:17:36,"David Furman" <sirok4 at gmail.com> wrote: Hello dear users and developers, I'm trying to calculate the equation of state of Europium oxide, but I have some doubts regarding my procedure. I have the xyz coordinates of the unit cell in angstroms which I put into atom_positions and also define the needed celldm's(). I'm interested to compress the material to different values (say, 0%, 10% and 20% volumetric compression) and calculate the total energy each time. Do I have to change the given initial xyz coordinates in atom_positions AND the celldm() , or just change the celldm() accordingly to the compression and pw.x will do the proper changes to the positions of the atoms ? I have done some calculations with uncompressed unit cell and with 20% compressed unit cell (by changing to the proper compressed celldm() values) but the difference in energy was about 0.5 Ry which i believe to be quite small. (Of course I checked previously for convergence). Am I doing it wrong ? Any help will be greatly appreciated David Furman, Graduate Student| Fritz Haber Research Center for Molecular Dynamics, The Hebrew University of Jerusalem | +972-8656-8909 -------------- next part -------------- An HTML attachment was scrubbed... URL: http://www.democritos.it/pipermail/pw_forum/attachments/20120609/d4fdf803/attachment-0001.htm