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QuantumATK => General Questions and Answers => Topic started by: freshgirl203 on July 23, 2009, 05:34

Title: Whats the difference between band structure energy and toal energy
Post by: freshgirl203 on July 23, 2009, 05:34

Part of my geometry opt output. Ebs(in red), according to the reference, meaning "band structure energy, i.e. the sum of the energies of all states weighted with their respective occupation". Then what is the meaning of "Total Energy"? Why these two values are different? Here, Ebs = -2957.3953 eV, while Total Energy = =-14817.9986 eV

# sc 21 : Fermi Energy =   -0.32537 Ry Ebs = -217.36284 Ry   dRho =  9.0874E-05
# Geometry optimization step : 69
# Total Energy           =-14817.9986 eV

Title: Re: Whats the difference between band structure energy and toal energy
Post by: zh on July 23, 2009, 09:28

Quote from: freshgirl203 on July 23, 2009, 05:34

Part of my geometry opt output. Ebs(in red), according to the reference, meaning "band structure energy, i.e. the sum of the energies of all states weighted with their respective occupation".

Yes, it is true.

Quote

Then what is the meaning of "Total Energy"? Why these two values are different? Here, Ebs = -2957.3953 eV, while Total Energy = =-14817.9986 eV

Why do you believe that the band structure energy should be same with the total energy? Although in the DFT calculation on the bulk or molecule system, the Kohn–Sham total energy can be written as a sum of a band-structure (BS) energy plus some correction terms, sometimes called 'double-count' corrections. The detailed formalism of Kohn-Sham total energycan be found in J. Phys.: Condens. Matter 14 (2002) 2745–2779 for QuantumWise package and SIESTA code.  While for the two-probe system handled by DFT+NEGF, the detailed formalism for band structure energy can be found in Phys. Rev. B 63, 245407 (2001).