[Pw_forum] Charge ordering with QE

[Mulazzi Mattia]

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