[Wien] Force Convergence Criteria/It-number

[Gerhard Fecher]

You should minimize the internal parameters BEFORE you apply the displacements
(does your initial structure have any free internal parameters at all ?)

The forces is what you like to have to calculate the phonons, what would happen if they would be zero?
The structure with displacement, indeed, will have free parameters but youre displacement is intended
and you should not go to change that if you like to calculate the phonons.

If you have free internal parameters the forces should indeed be non-zero, you minimize the forces you do not make them vanishing.

Remember: minimization of the energy does not make it zero, I guess your total energy is very much different from your convergence criterion.

Ciao
Gerhard

====================================
Dr. Gerhard H. Fecher
Institut of Inorganic and Analytical Chemistry
Johannes Gutenberg - University
55099 Mainz
________________________________________
Von: wien-bounces at zeus.theochem.tuwien.ac.at [wien-bounces at zeus.theochem.tuwien.ac.at]" im Auftrag von "Judy Cherian [judy.cherian at gmail.com]
Gesendet: Dienstag, 9. August 2011 05:12
Bis: Wien at zeus.theochem.tuwien.ac.at
Betreff: [Wien] Force Convergence Criteria/It-number

Hello all,

I am doing SCF calculation on displacement files created by phonopy.( Displacements files were made using a p-type supercell created from struct file after performing Force convergence and Volume optimization with minimization of internal parameters). There are 44 atoms in the unit cell. When doing the SCF calculation using w2web interface, I chose the 0.01 mRy/a.u as the force convergence requirement. The calculation ends with the day file telling that force convergence was met. However, when I look at the total force at the end of .scf file, the force on each atom is much higher that 0.01 mRy/a.u. I was wondering how SCF determines when the force convergence is met. Is it by the Total Forces on each atom , or something else?

Also, how can I increase the number of iterations in scf cycle? Does checking the It-number in w2web Interface ( in the Run SCF menu) and changing the value from the default value of 40 to some other value increase the number of iterations? Is there a limit on the iteration number?

Thank you

Judy Cherian
Graduate Student
NHMFL/FSU
Tallahassee-Florida