If a negative (i.e. imaginary ) frequency is present at a non-zero q point this means that the system is unstable w.r.t a displacement at that q vector and therefore the unit cell for the (static) equilibrium geometry should actually be larger than the one you have chosen and should such that the unstable q-vector folds into the Gamma point of the super-cell. This is indeed the case for bcc Zr where if I remember correctly the bcc phase is stable only at high temperature due to anharmonic effects [see for instance PRB 31 6775-6778 (1985) and PRL 58, 1769-1772 (1987) by Chen, Fu, Ho, and Harmon]
Stefano de Gironcoli - SISSA and DEMOCRITOS Prasenjit Ghosh wrote: > Dear Partha Sarathi, > > I am not an expert on phonon calculation and I am also not familiar > with your system; however, an imaginary (negative) phonon mode at a > non zero q-vector usually indicates that the cell parameters (crystal > lattice vectors) which your determined, does not correspond to the > equilibrium one. Have you checked how the stresses are with your > equilibrium geometry? > > Also at gamma point, are you getting 3 zero modes and rest non-zero modes? > > With regards, > Prasenjit > > On 21 May 2010 11:41, partha sarathi ghosh <parthasarathi13 at gmail.com> > wrote: > >> Dear all, >> Thank you xirainbow and Michael J. Mehlfor for >> your reply & suggestion . >> I was trying to calculate longitudinal phonon dispersion of Zr along [111] >> direction. >> I wanted to validate my calculation with experimental results for bcc-Zr >> (not hcp) given in the paper :Phy.Rev.B,vol-29,page 1575-1587(1984). >> But I am getting negative frequencies near q=2/3(1 1 1) and the over all >> appearance of the curve is different from the experimental one. I optimized >> the system w.r.t ecutwf, ecutrho , k-point sampling , volume ,degauss, >> conv_thr. I have also tried with different q-point sampling in ph.in file >> (like 4 4 4, 8 8 8) with tr2_ph=1.0d-12. >> My questions are : >> 1. Is DFTP suitable for studying particular phonon mode like in Zr ?? >> 2. If yes then how to make my calculations efficient ?? >> 3. If not then any suggestion of other methods ?? >> Any one can please help me. >> Thanks in advance. >> P. S. Ghosh >> B.A.R.C. >> INDIA >> >> _______________________________________________ >> Pw_forum mailing list >> Pw_forum at pwscf.org >> http://www.democritos.it/mailman/listinfo/pw_forum >> >> >> > > > >