I was trying to calculate frontier orbital's energy for benzene. But as per gaussian software result with HF/3-21G, Alpha occ. eigenvalues -- -0.29755 and Alpha virt. eigenvalues -- 0.09401. Then how much is Energy of HOMO and LUMO?
Dear Devi,
The last value in the Alphha Occ. eigenvalues gives the HOMO energy and the first value in the Alpha Virt. eigenfunction gives LUMO energy.
The value of HOMO is always higher than LUMO but is a negative value. In your example, HOMO= -0.29755 an LUMO= -0.094. The Band Gap is -0.094-(-0.29755)= about 0.2 eV.
Hoping this will be helpful,
Rafik
Hello,
in order to get the energies of HOMO and LUMO in eV, you should multiply the obtained values, which are in Ha, or A.U., by 27.212, which is the convergence factor from A.U. to eV. Another point: the approach which you use is really bad for calculating MOs energies, HF usually gives strongly overestimated HOMO-LUMO gaps. Much better for this purpose are the DFT methods, with larger basis sets. For reference, please look here:
http://cccbdb.nist.gov/gap2x.asp
Best regards,
Aleksey.
Hello, you can use following energy conversion factors-
http://www.ucl.ac.uk/~uccaati/Energy.html
Or,
Energy unit converter
http://www.colby.edu/chemistry/PChem/Hartree.html
Devi Sree
First optimize the molecule then compute electronic transitions and finally check the difference between the eigen values of LUMO and the HOMO. (Energy difference will be in Hatree).
The last value in the Alphha Occ is gives the HOMO energy and the first value in the Alpha Virt. eigenfunction gives LUMO energy.
exp.
Alpha occ. eigenvalues -- -0.29481 -0.27556 ( -0.26765)----Homo
Alpha virt. eigenvalues -- (-0.06849) LUMO -0.01261 -0.00504 0.00443 0.01166
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