Solution LAB2 guanine 3
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For calculation of E(N-1) we need to set the following flags in the &SYSTEM namelist:
nspin=2 ! Note, now we have unpaired electrons: tot_charge=+1 ! charged system tot_magnetization=1 ! we need to set a total magnetisation
Also, the following option needs to be used for charged systems :
Using ecutwfc=50 Ry and 35 au of cell size,
E_tot(N) = -193.48101821 Ry E_tot(N-1) = -192.89782124 Ry
IP = E_tot(N-1) - E_tot(N) = 0.58319697 Ry = 7.934861 eV
now much closer to the experimental value of 8.0-8.3 eV.
Interestingly, one can run the same set of calculations also using different functionals, including GGAs (PBE), meta-GGAs (eg TPSS or SCAN), hybrids (PBE0), or HF itself.
In order to do this, you can force e.g. the functional from input, by using:
&SYSTEM [...] input_dft="PBE0" /
How do -eps_HOMO and delta-scf values compare ?