[Wien] volume optimization and force minimization

[Laurence Marks]

Unless something has changed, forces are not implemented for
spin-orbit correctly so you cannot do a force minimization with them
(unless you do some tricks, i.e. minimize the enegy by hand not using
the code).

For spin/U the question is how large a difference does this make to
the electron density -- if it is large then the positions/optimal
volume will be very different.

But...do not depair! If spin-polarized and U is a much better
description of the physics and you do not have multiple magnetic
states, convergence of a LDA+U (or GGA+U) calculation can be much
faster than a LDA calculation which does not describe the physics
well. This follows from the physics/math of mixing.

N.B., this is also why using a small mixing term in a
Broyden/Multisecant method does not do what most people think it does,
conventional wisdom is incorrect or at most correct for Pratt mixing.
(Also, large mixing terms do not do what people think they do!) In
general this not the way to solve a badly convergent problem, the
correct (only) solution is to improve the physical model.

2010/2/24 shamik chakrabarti <shamikphy at gmail.com>:
> Dear Wien2k users,
>                             I have a question regarding volume optimization
> and force minimization.
> If I do volume optimization and force minimization for non magnetic
> calculation and then use that optimized volume and structural coordinates as
> input for the calculation of volume optimization and force minimization
> including spin polarization, spin orbit coupling term and LDA+U term, then
> whether I get the correct optimized values for both volume and coordinates
> of the structure which really need those corrections?
> If I want to do volume and coordinates optimization including those
> correction terms from the begining then it will take a very long time. But
> if we do the optimization using non magnetic calculation to reach closer to
> the actual energy and force minima and then put those optimized structure
> for non magnetic calculation for the optimization of the structure having
> those corrections it may take lesser time to reach the actual minima. But
> whether we really get actual minima in this way?
> The above two questions actually are the same questions. I am very sorry for
> that reapeating. Actually I want to make clear my intentions to the wien2k
> users.
> Thanks in advance.
> regards,
> Shamik Chakrabarti
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>



-- 
Laurence Marks
Department of Materials Science and Engineering
MSE Rm 2036 Cook Hall
2220 N Campus Drive
Northwestern University
Evanston, IL 60208, USA
Tel: (847) 491-3996 Fax: (847) 491-7820
email: L-marks at northwestern dot edu
Web: www.numis.northwestern.edu
Chair, Commission on Electron Crystallography of IUCR
www.numis.northwestern.edu/
Electron crystallography is the branch of science that uses electron
scattering and imaging to study the structure of matter.