[CP2K-user] [CP2K:20344] HOMO-LUMO gap vs band gap from PDOS

[Arup Sarkar]

Hello all CP2K users,

I have a very basic query related to HOMO-LUMO gap calculation in CP2K 
(9.1). I am trying to calculate the HOMO-LUMO gap and also partial 
density-of-state (PDOS) of a nanocluster. First I optimized the geometry 
using periodic boundary conditions by putting my large nanocluster inside a 
large box so that no interaction occurs with its periodic image. Then, I am 
calculating the ENERGY with HOMO-LUMO settings using the box size and 
POISSON solver XYZ or Periodic. 

I am getting the same HOMO-LUMO gap even I use non-periodic boundary 
conditions with POISSON solver wavelet with the same input settings. Now, 
when I am trying to calculate the energy gap or more precisely the band gap 
from PDOS calculation (of course using periodic boundary conditions at 
gamma point) with smearing (and also without smearing) and with OT (and 
also without OT),  the band gap is less than the HOMO-LUMO gap from the 
previous calculations. 

Now it is known that, band gap from band or DOS calculation is usually 
considered smaller than the cluster HOMO-LUMO gap but this is true if my 
HOMO-LUMO calculation is performed in non-periodic mode. Why the periodic 
HOMO-LUMO and non-periodic HOMO-LUMO gap matches in CP2K ? Is it converting 
my calculations automatically to a non-periodic calculation even I mention 
POISSON solver periodic during the HOMO-LUMO energy calculations ?

  With regards,
Dr. Arup Sarkar
Postdoctoral Researcher
Max Planck Institute for Polymer Research

 


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