On Mon, 5 Jun 2006, bipul rakshit wrote: | hi siesta users, | i am calculating the phonon frequencies of MgB2. i got nice match in bulk modulus of 156 GPa with LDA. While the experimental value is 151 GPa. | | With that Pseudopotential when i tried to calculate the phonon frequency by Vibra. i got -ve frequencies..... | | can any body suggest, what can i do???
Bipul: You must obtain three acoustic modes with the frequencies close enough to zero, as the lowest eigenvalues. If they are ~ 0.01, either positive or negative, that's fine. If any of them largely deviates from zero, that is a indication that are not good because you have eggbox effect, as a result the acoustic sum rule is not satisfied with sufficient accuracy. You can check the eggbox effect prior to phonon calculation: displace all atoms uniformely, look at the variation of forces; the sum of forces over all atoms in each cartesian direction must be less than, at most, 0.1. Increase Mesh Cutoff to control the eggbox. Good equilibrium lattice constant and bulk modulus are good indication that the calculation is generally sane; however they are not so sensitive to the eggbox as the forces (and hence phonons) are. Good luck, Andrei +-- Dr. Andrei Postnikov ---- Tel. +33-387315873 ----- mobile +33-666784053 ---+ | Paul Verlaine University - Institute de Physique Electronique et Chimie, | | Laboratoire de Physique des Milieux Denses, 1 Bd Arago, F-57078 Metz, France | +-- [EMAIL PROTECTED] ------------ http://www.home.uni-osnabrueck.de/apostnik/ --+
