Hi there,
I wish to perform a TD-DFT calculation on an insulating metal-organic system (fundamental gap of ~5.5 eV) to obtain the optical gap of the material. I am using the DSH functional (parameterized according to: https://pubs.acs.org/doi/10.1021/acs.jpclett.8b00919). I have used the following INCAR for the ground state calculation:
EXX = 0.52
ALGO = Conjugate
EDIFF = 1e-06
HFSCREEN = 1.89
ISMEAR = 0
LHFCALC = .TRUE.
LMODELHF = .TRUE.
LORBIT = 11
LREAL = .FALSE.
NEDOS = 5000
NELM = 1000
NSW = 0
NWRITE = 1
POTIM = 0.4
PREC = Accurate
SIGMA = 0.1
SYMPREC = 0.001
I understand from the wiki that I should now do a separate calculation where I include more unoccupied states and LOPTICS = .TRUE. to obtain the WAVEDER file required for the following TDHF step (https://www.vasp.at/wiki/index.php/Time ... lculations). Then, after the ALGO = TDHF calculation, I should have a resulting optical spectrum. I wanted to get some clarification, since I noticed a separate tutorial (focused on comparing TDDDH with GW/BSE) an exact diagonalization step (ALGO = Exact) was required for the unoccupied states (https://www.vasp.at/tutorials/latest/bse/part1/). Is this just because the system is sufficiently small that exact diagonalization is easily tractable, or should I be including this step? Perhaps I am confused about why exact diagonalization is required in the first place. Any suggestions on the correct procedure considering my system and desired property would be really appreciated.
Thanks!
Jake