[QE-users] Difference between Geometry Optimization and a SCF calculation

[Giuseppe Mattioli]

Dear Chapman Gupta
The three calculations are all reliable, but they have different  
purposes; you should know what you need.

scf: single point calculations (possibly with calculation of forces  
and stresses at the end). It gives you the total energy and  
eigenvalues @ k-points of the input configuration

relax: Hellmann-Feynman forces on nuclei are calculated at the end of  
each scf step and nuclei are displaced accordingly in order to find a  
local energy minimum by using the bfgs procedure. NB if the system is  
highly symmetric (e.g., if you place Si atoms in their proper lattice  
positions) then the resultant of forces is zero. At the end of the  
calculation the code gives you the total energy and eigenvalues @  
k-points of the optimized configuration

vc-relax: lattice parameters and nuclei positions are optimized in the  
same run, according to forces and stresses and depending on the  
required lattice symmetry. At the end of the calculation the code  
gives you the total energy and eigenvalues @ k-points of the optimized  
configuration in an optimized cell.

HTH
Giuseppe

Quoting Chaman Gupta <chaman at uw.edu>:

> Hey everyone,
>
> Might be a very naive question, but I have been very confused with this
> recently.
>
> I have a 'Si - simple hexagonal' cif file. I want to see the band structure
> and/or the band gap (HOMO - LUMO).
> 1. I ran a 'scf' calculation on BURAI (a GUI for QE). It generated an
> output file which gave me the Total energy and the energy of HOMO and LUMO.
> 2. Now separately, I ran a 'geometry optimization' calculation (both
> 'relax' and 'vc-relax': thus 2 different calculations), again on BURAI, and
> I got an output file, which gives me Total energy and energy of HOMO and
> LUMO.
>
> *I specified nbnd = 8 (no. of bands) for both the above mentioned
> calulations*
>
> I am confused as in which one is correct or more reliable. And also when
> should we use which calculation?
>
> Help is really appreciated.
>
> Regards,
>
> *Chaman Gupta*
> Graduate Research Student, *Novosselov Research Group
> <http://depts.washington.edu/nrglab/>, UW*
> Graduate Research Student, *Pauzauskie Research Group
> <https://depts.washington.edu/pzlab/wordpress/>, UW*
> University of Washington Seattle, *M.S. in Mechanical Engineering*
> IIT Kharagpur, *B.Tech in Metallurgical and Materials Engineering*
> *Linked In <https://www.linkedin.com/in/professorchamangupta> | Email
> <chaman at uw.edu> | **P*: +1 (206) 383-3514



GIUSEPPE MATTIOLI
CNR - ISTITUTO DI STRUTTURA DELLA MATERIA
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E-mail: <giuseppe.mattioli at ism.cnr.it>