On Wed, 29 Oct 2008, JR Schmidt wrote: JRS> Is there an simple mechanism that can be used to estimate a reasonable JRS> cutoff energy for a given pseudo-potential? I realize that these values JRS> need to be checked for convergence, but I would like to at least get a JRS> reasonable starting point. It appears that there are fields stored in JRS> the pseudo-potential files that are supposed to give "reasonable" JRS> defaults for both the wavefunction and density KE cutoffs, but they seem JRS> to be always zero in every case I have checked.
yep. these cannot be easily detected automatically. one can derive some estimates during the generation procedure though and UPF files generated in version 2 file format contain this value. however, different properties converge differently with the size of the basis set (wfc and density cutoffs). and obviously nobody cared to edit the old files. in general there are some rough guidelines: ultra-soft pseudopotentials require usually about 30-50 ry wavefunction cutoff and 6-10 times the density cutoff. norm-conserving pseudopotentials generally require higher wavefunction cutoffs, say, 60-120ry sometimes even more, sometimes (e.g. Si) less and there is no gain from changing the density cutoff. forces generally converge faster than total energies or the stress tensor. energy differences also tend to converge fast. JRS> I'm guessing a possible answer is to run a calculation on the isolated JRS> atom for each atom type. However this seems like something that would usually a good estimate is done with a small bulk system with a proper k-point sampling. checking the E/V curve for different cutoffs is a frequent test for pseudos. JRS> have been done previously for each pseudo-potential and tabulated somewhere. feel free to start such a table. JRS> Please forgive me if this question comes up frequently, but I couldn't JRS> find an easy mechanism to search the archives. you can (ab)use google. adding "site:democritos.it" to your query will search only the server where the mailing list is currently archived. there also is a FAQ on the wiki at http://www.quantum-espresso.org cheers, axel. JRS> JRS> -- ======================================================================= Axel Kohlmeyer akohlmey at cmm.chem.upenn.edu http://www.cmm.upenn.edu Center for Molecular Modeling -- University of Pennsylvania Department of Chemistry, 231 S.34th Street, Philadelphia, PA 19104-6323 tel: 1-215-898-1582, fax: 1-215-573-6233, office-tel: 1-215-898-5425 ======================================================================= If you make something idiot-proof, the universe creates a better idiot.
