Dear developers,

I'm calculating spin-flip excitons with yambo by including spinor wavefunctions in DFT step (lspinorb=.true. in qe). The material has a Mott-insulator-like band structure at the band edge, so I simply used one spin-up valance band and one spin-down conduction band to construct the BSE Hamiltonian in the calculation, and I thought it should produce merely spin-triplet excitons. Then I try the function "-e spin" in ypp. It gives two files "exc_E+spin" and "exc_I+spin" which contain exciton eigenvalues and are quite similar to those created by "-e s", but with two additional lines in each file dubbed as "S_z" and "S_square". However, I found that all the values in the two lines are "0.000", although there are only spin-triplet S=1 excitons herein.

So I think the "spin" here doesn't refer to the total moment of exciton, but something like an averaged spin value among the associated electrons and holes. Is that the case? Would you please tell me the exact definition of "exciton spin" given in the file?

Thanks a lot,