Hi,
I'm calculating the Hubbard U for Fe (octahedral) and Fe (tetrahedral) in bulk Fe3O4. The ultrasoft pseudopotential (USPP) I'm using now was downloaded from QE website called "Fe.pbe-nd-rrkjus.UPF". I'm using QE-5.0.2-pgi-13.4 But I got inconsistent U(Fe,oct) and U(Fe,tet) values when I use different type of cell to do the linear perturbation. When I use cubic cell (56 atoms in total, 16 Fe_oct, 8 Fe_tet and 32 O), I got U(Fe,oct) < U(Fe,tet). When I use hexagonal cell (42 atoms in total, 12 Fe_oct, 6 Fe_tet and 24 O), or rhombohedral primitive cell (14 atoms in total, 4 Fe_oct, 2 Fe_tet and 8 O), I got U(Fe,oct) > U(Fe,tet). I changed the CELL_PARAMETERS to make the Fe_oct and Fe_tet in cubic cell have the same Fe-O bond directions as those in hexagonal or rhombohedral cell. But still, it gave U(Fe,oct) < U(Fe,tet). I don't know if the problem is from the USPP or from the linear perturbation theory itself implemented in QE. People keep treating these two types of Fe the same in magnetite Fe3O4 studies. I read papers from Emily Carter's group using different U(Fe,oct) and U(Fe,tet). But their U values were not derived by calculation or by Fe3O4 itself, but from empirical combination obtained by FeO and Fe2O3. I got stuck with those inconsistent U values of Fe3O4 from QE and have no idea about how to deal with it. Thank you, Xu Huang
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