Dear QE community, I am currently studying charge-density wave ordered materials form the theoretical point of view. In my system, VS2, the Vanadium planes order in an hexagonal 2D lattice and are sandwiched by two Sulfur planes. As a basic approach, an hexagonal unit cell with one V and two S atoms produces a band structure that is in fair agreement with the experimental data and is consistent with other all-electron calculations. But both theoretically and experimentally there are hints that the system is orbital-ordered, i.e. that the d electrons on the Vanadium sites occupy orbitals of different symmetry on different Vanadium sites. In order to calculate the electronic structure of an orbitally-ordered system I think I have to use a supercell and specify different occupation numbers for the different Vanadium sites. The problem is that I don't know how to do that. I tried specifying the starting_ns_eigenvalue variable on different sites, but the converged calculation is identical to the calculation with starting_ns_eigenvalue left unspecified. In the INPUT_PW it is written that the occupation matrices can be specified only when the LDA+U scheme is used. However, I cannot use the LDA+U on my system (Mac OS X 10.5.6) because every time I use the Vanadium pseudopotential (either the one I generated, ether the one on the QE database) I find a "segmentation fault" that I cannot solve. BTW, the segmentation fault error does not appear for Co or Fe or Ni pseudopotentials.
Thanks everybody for the help, Mattia Mulazzi FPR Fellow of RIKEN at Spring8 Excitation Order Research Team 1-1-1 Sayo-cho Sayo-gun, Hyogo Japan