Zhen, Again I ask you: what k-point sampling are you using in your calculations? The reason I am being pushy on this is because I am very familiar with an extreme case, which is graphene. You can get very reasonable results when you perform calculations for graphene with a low k-sampling, if you enforce symmetries - for example, setting
LatticeConstant xxxx Ang %block LatticeVectors 0.5 0.866025404 0.0 -0.5 0.866025404 0.0 0.0 0.0 10.0 %endblock LatticeVectors %block kgrid_monkhorst_pack 9 0 0 0.0 0 9 0 0.0 0 0 1 0.0 %endblock kgrid_monkhorst_pack but when you do a variable-cell calculation, in the end, you will find a sizeable deviation, in the angle between the in-plane lattice vectors, from the expected 60 degrees. It only goes away when you set a pretty high value for the MP grid, such as 21x21x1. Being extremely pushy, what you should do is convergence studies in your parameters, if you still haven't done so. It could well be that some parameters in your calculation are not good, which happens often when systematic convergence studies are not performed. Please disregard this if you have performed said studies, but contact you supervisor if you don't know how to perform one - he/she should be able to teach you how to do this. If not, let us know. It could be that, as Derek states, you could need semicore states or non-linear core corrections to be included in some cases. It would be advisable to check on the literature if this is the case for your metallic Ti. Now, one correction. Phonons in siesta are not calculated by frozen-phonon calculations, although siesta (and any other ab initio software, for that matter) can perform frozen-phonon calculations. What siesta does is a calculation of the force constant matrix in real space by finite differences. Then the phonon frequencies are calculated by diagonalization of the FC matrix. Now the following wild guess occurs to me... Your phonon softening seems to occur for large values of the phonon wavevector, q, and for small values it's pretty well-behaved. Could it be that your supercell is not large enough to support such a mode, and then it gives you this spurious softening? Phonon experts on the list, am I just guessing it wrong? Marcos On Thu, Mar 4, 2010 at 4:54 PM, Zhen Huang <[email protected]> wrote: > Dear Derek and folks- > Thanks for you responds. I have tried ABINIT LDA from SIESTA website and it > also give very small lattice constants without any treatment to spin (2.67 > Ang comparing to 2.95 Ang from textbook). Further, the one of the angles > changed from 120 to 108 degree. I could not tell what is the problem is. I > have used PWSCF to obtain phonon dispersion of Ti before and it worked out > fine. Although, I have never gone into the details of Isaev's paper. > The reason I switch to SIESTA, is the phonon calculation is based on frozen > phonon and when it comes to large system it is much faster than Quantum > espresso. However, seems at this point, I am not able to get the Ti running > properly. > Best Regards > ------- > Zhen (Alex) Huang > Ph.D. Student > Nanoscale Transport Research Group > Laboratory for Computational Methods in Emerging Technologies > Cooling Technologies Research Center > School of Mechanical Engineering > Purdue University > Tel: 765 237 9733 >
