## dynmat calculation to get prefactor for vacancy jump

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### dynmat calculation to get prefactor for vacancy jump

First of all, thanks for kindfully providing your codes. They have really made my work much easier. So far I have been doing quite a few CI-NEB calculations to obtain the energy barriers for diffusion which worked fine. More recently I tried to obtain prefactors via the Vineyard equation but encountered problems when trying to analyze the dynamical matrix calculations: They gave several imaginary frequencies for the initial state as well as the saddle point configuration. I repeated the calculations for a perfect cell and still got one imaginary frequency. I tested displacements between 0.001 and 0.1A and varied the size of the dynamical matrix between 15x15 and 93x93 (with 31 atoms in the full cell). I also fitted several matrices corresponding to different displacements. In all cases I got imaginary frequencies. The system of interest is a vacancy jumping on a wurtzite lattice. Currently, I am using 32-atom supercells. Larger supercells would be desirable but are beyond the computational capacity at my disposal. What else do I have to check? Could the problem be related to the lattice symmetry? Any hint would be appreciated. Thanks.

Also, make sure you have a small ediff. Try ediff=1e-8 to get accurate forces.

You minima should be well converged, using at least ediffg=-0.01.

Make sure that the masses are correct in your massfile.

One last issue could be the smearing function. I had problems trying to find the frequencies of CH4. It turns out that the default M-P smearing was causing problems and the Gaussian smearing works better. This will show up as something odd in the density of states. There was a helpful message about this on the vasp forum:

http://cms.mpi.univie.ac.at/vasp-forum/ ... .php?4.138