Dear Paul,
> 1.) What kind of influence a semicore state in the pseudopotential has on the > calculation. I found some general information of > the comparison of norm-conserving and non norm-conserving (ultrasoft) > pseudopotentials but I don't find any good > written description of the influence of semicore states. Which > properties are altered the > most if using semicore states and what are the advantages and > disadvantages? in general you can have semicore states both in the norm-conserving (nc) and ultrasoft approaches (us). The only disadvantage of including them is the additional computational cost (more electrons, and more structured orbitals that require a higher cutoff). sometimes the "non-linear core correction" can be used (e.g. in group I/II) to account for core/valance overlap - see the 1982 Louie PRB, or http://www.tcm.phy.cam.ac.uk/~mds21/thesis/node17.html Let's take Fe: 4s2 3d6 are the valence, while 3s2 3p6 are the semicore. 8-electron pseudos are probably good to describe metallic iron, but not much else - but only a comparison of calculations done with or without semicore can tell you if they are needed (if they make a difference, they are needed). Adding 3p6 (at the cost of 6 more electrons) is probably a very good thing, while adding 3s2 (although customarily done) probably doesn't help that much (they are too low in energy). A critical difference here emerges between us and nc pseudo - nc pseudo have one projector per angular momentum, while us have typically two - hence if you use the 3s semi-core, with us you can have a s projector at 3s energies, and a s projector at 4s energies, while with nc you are forced to have one projector that acts in the same way on the 3s and 4s (a bad idea - better to drop the 3s back in the core). > 2.) Since the systems I have to deal with contain heavy elements the question > arises what kind of influence the spin orbit > coupling has on structural and electronic properties. I found one > paper and the two corresponding potentials for gold > and platinum which incorporate a full relativistic treatment and > enable a calculation including spin-orbit coupling for > the LDA functional. Perhaps I havn't found the example of tutorial but > is there a general procedure to incorporate > SO effects on a GGA or a meta-GGA level yet? I'd look at Andrea Dal Corso papers on phonons - probably the effects on structural properties are small, and on electronic properties, ahem, relevant for all properties that are not there without spin-orbit :-). Do keep in mind, though, that the pseudopotentials themselves have been generated with a relativistic calculation for the all electron atom, so a lot of the key effects have been captured there already. > > A third question is dealing with the density cutoff. > > 3.) In the QEwiki is written that a higher density cutoff should be used in > the case of ultrasoft pseudopotentials and it should > be checked that all setting have converged. I have done some > calculations and changed first of all the Ecutoff and after > that the Rhocutoff from four to ten times the Ecutoff. There was no > big influence of the Rhocutoff on the structure and energy > in my particular case. Just to avoid a problem in the future. What > properties are altered the most by using a insufficent Rhocutoff? > Best way to think at this is not that a higher density cutoff is needed (density cutoff is fixed by the true physics of the charge density), but a lowe wavefunction cutoff can be used. The end result is the same. 8 times is a good ballpark, and 6 to 12 the common range. Of course, if your Ecutoff is very high, then 4 will work - but the point is that you want to use for both the minimum value that is sufficient. A good check can be optical phonons - do them with energy cutoffs of 10-12-14....-38-40 Ry, using a rhocutoff of 4, 6, 8, 12 times , and compare these curves. nicola > Best wishes > > Paul > > > -- ---------------------------------------------------------------------- Prof Nicola Marzari Department of Materials University of Oxford Chair of Materials Modelling Director, Materials Modelling Laboratory nicola.marzari at materials.ox.ac.uk http://mml.materials.ox.ac.uk/NM
