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.