[Wien] Concerning use of pairhess and case.inpair

[Laurence Marks]

I want to post a little extra information about the latest version of
pairhess and eigenhess, for reference.

When you run pairhess, you will see an output such as:

 case.inM present and used for constrains
 Average Hessian Eigenvalue    453.6 mRyd/au^2, Frequency  542.18 cm-1
 Min & Max of Eigenvalues, mRyd/au^2     48.2   851.8
 Min & Max frequencies, cm-1   177   787
 Check .minpair, the estimate, and output in
 /home/ldm/Wien/MgO_111/Oxides/Octa/Octa.outputpair
 PairHess END

If you already know roughly what the average vibration frequencies of
you system are, you can adjust RESCALE in case.inpair (as described in
the User Guide) to an appropriate value. For instance, in the above I
used 0.45 which based upon previous work I guessed as being closer for
MgO than the default 0.25. (By looking at the frequencies from a
different run from eigenhess). I would suggest adjusting the default
if you are experienced -- it may save several minimization steps.

What eigenhess will provide is an estimate of the symmetry-allowed
vibration frequencies. Technically speaking this estimate will
converge to the true values if the minimization is allowed to run to a
high degree of accuracy (perhaps forces of 1 mRyd/au or less),
although it can be a little off if the forces are larger. The option
"XYZ" in case.ineig is particularly useful as it will produce a file
that can be read using JMOL which can display the vibrations. Minimal
details of the relevant options are provided at the start of
case.outputeig.

-- 
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.