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 :

assume_isolated="mt"

Using ecutwfc=50 Ry and 35 au of cell size,

 E_tot(N)   = -193.48101821 Ry  
 E_tot(N-1) = -192.89782124 Ry

giving

 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 ?