Emma, the suggestions done by Alexander on your problems seem very reasonable. I'd address some more points.
1. In the pseudopot generation, you use Co 3s2 3p6 3d7 i.e. for the Co2+ case, and the basis functions 3s(SZ) 3p(SZ) 3d(DZ) 4s(DZ) 4p(SZ) I wonder if you really need 3s, 3p as valence state. You may wish to include them in a FLAPW code (along with I4d) when this would cost you nothing, but in a pseudopotential calculation you have to decide which are your valence states needed for the chemical bonding, and I think 4s is much better bet. This will give you a much less problematic pseudopotential. In fact Co3s are too deep to have an effect on anything. Co 3p are not so critical, you can include them in the valence band if you wish, but this won't be really needed, because they are much lower than I5s. I think you could make better pseudopot with, say, 4s1 3d8 configuration; with core correction. For iodine, the pseudopot configuration seems reasonable, but I don't know about the details, radii etc. In the basis, I don't think you need 6s and 5d, better to add polarization orbitals on 5s and/or 5p. 2. Apart from this, there must be a minimum of total energy vs. volume, if you have good overlap of basis functions. But this minimum might be quite apart from the experiment, due to problems with pseudopots/bases. In your system, I'd expect the compressibility to be very different in the XY-plane and with respect to Z-compression. Try to explore the energy minimum within a large enough interval of varying a, and then c, or just start an automatic relaxation as Alexander suggests. 3. For the true relaxation, you'd need low force sum, that means higher EnergyCutoff. This will however depend on the choice of pseudopotentials. Best regards, Andrei +-- Dr. Andrei Postnikov ---- Tel. +33-387315873 ----- mobile +33-666784053 ---+ | Paul Verlaine University - Institute de Physique Electronique et Chimie, | | Laboratoire de Physique des Milieux Denses, 1 Bd Arago, F-57078 Metz, France | +-- [EMAIL PROTECTED] ------------ http://www.home.uni-osnabrueck.de/apostnik/ --+ On Thu, 9 Nov 2006, Emma Sceats wrote: | Andrei | | Thanks for taking the time to respond to my email. | | I must apologise for a couple of typos in my email to you. First off, I meant | to say that CoI2 crystallizes in P-3m1 (space group 164) rather than P3-m1. My | coordinates should have read: | | Co 0 0 0 | I 1/3 2/3 0.25 | I 2/3 1/3 0.75 | | (oopps - big typo there!) | | I specify the lattice constant (3.96 Ang) and the Lattice parameters as: | | 1.0 1.0 1.679 90 90 120 | | I have attached the pseudopotentials and output file of a calculation. I | should say that I have successfully used the Co pseudo and basis set in | calculations on CoO (I chose this as a test system since there is lots of data | (both experimental and calculated) on the structure, bulk modulus etc and it | is quite similar (in terms of level of covalency) to the CoI2 system I am | interested in). Also the I pseudo and basis were used in some calculations I | ran on KI (and gave a good lattice const and bulk modulus). | | A couple of things to point out: | (1) the forces on the atoms in the unit cell are HIGH (0.9 eV/Ang) but only in | Z. | | (2) when I vary the lattice parameter slightly (as if I was calculating the | cohesive curve to get the bulk modulus) I do NOT see the expected parabolic | dependence of the energy on the lattice constant (with a minimum at the | equilibrium lattice constant). Instead I see an increase in the energy with | increased lattice constant. (although I do see an approximately linear | decrease in the pressure with increasing lattice constant). | | I am completely confused as to why I see these effects, and emailed the | mailing list originally because I thought I had done something wrong with my | input coordinates. | | Can you check the coordinates and lattice parameters are consistent (I was a | little confused with what you thought I should change) and if they are OK have | you any other suggestions you can offer? (changing the | pseudos/basis/coordinates/lattice vectors or parameters etc etc) Anything you | think I could try would be helpful - I am runing out of ideas!!! | | | Emma |
