On Thu, 16 Oct 2008, Hui Tang wrote: hi hui,
your cell is expanding quite a bit and your cutoff is not that large. with an expanding cell, your _effective_ cutoff becomes much smaller as the number of plane waves remains constant during a variable cell calculation. have you tested the stess tensor convergence vs. the cutoff? you basically have to figure out how many plane waves you need for a well converged stress tensor at the largest expansion of your cell and then you have to set the cutoff at the beginning so that it results in the same number of plane waves... that would explain why it works (better) if you restart. also, did you test the pseudopotentials for ghost states? cheers, axel. p.s.: please provide an affiliation. thanks. HT> Hi Nic, HT> HT> 1) I have tested the pseudo-potentials (psps) with small atomic HT> separations. The pseudo-potentials give reasonable till two atoms are as HT> close as 0.8 A. When the total energy starts to diverge, which is about HT> Ionic iteration 17, the smallest inter-atomic distance is about 1.4 A. So I HT> do not think the breakdown of the psps is the trigger of this divergence of HT> the total energy, although it should be the reason for the enormous large HT> negative energies. HT> HT> 2) I have tried to restart the calculation from where the divergence starts. HT> Once I did that, the first scf calculation would give a reasonable energy HT> around -41.7 Ry, and the calculation could run smoothly till the total HT> energy converges most of the times. So what I deal with this problem right HT> now is to restart calculations from where they start to have diverged total HT> energies and it's been working well. However, I hope this is not the optimal HT> way to solve this problem. :) HT> HT> Thank you very much, HT> Hui HT> -- ======================================================================= 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.
