Dear Carlo, as you don't specify how negative your negative frequencies are, it is not so easy to judge... Your sum of forces seems a bit too high... Even if not dramatically... And some individual forces as well - e.g., Fx on atom 2. So generally I'd recommend even higher MeshCutoff and further relaxation. A priori 350 Ry is not safely high, sometimes one needs to go to much higher.
Before doing the phonons, I'd recommend the usual "eggbox test" to be sure that fluctuations of summary force as function of uniform displacement of all atoms remain within |0.1|. Concerning specifically the phonons, you may wish to check http://www.home.uni-osnabrueck.de/apostnik/Lectures/APostnikov-Phonons.pdf - especially p.18 of it. Best regards Andrei Postnikov > Dear Siesta users, > > I am trying to calculate phonon frequencies at Gamma for a molecular > junction. > I relaxed the system by keeping fixed the Lattice Vectors (not a variable > cell). When I compute the frequencies (with MeshCutoff = 200 Ry), I obtain > that the first three frequencies are negative, which indicates that the > results are not correct. > Then, if I increase the MeshCutoff to 350 Ry (which seems a high value to > me) I obtain again negative frequencies. > Could you suggest me how to fix the problem? > The following are the forces of the system...do you think that they are > too > high? Which is a good criteria to say that the forces are low enough? > > Thank you very much, > > Carlo > > siesta: 1 0.106233 -0.096180 0.047641 > siesta: 2 -0.655838 0.034936 -0.004086 > siesta: 3 0.364736 -0.100368 0.166114 > siesta: 4 0.130690 0.010500 -0.101584 > siesta: 5 -0.122410 0.420815 -0.012044 > siesta: 6 0.120625 -0.031292 0.152270 > siesta: 7 0.003319 0.004051 -0.012274 > siesta: 8 0.004951 -0.015719 0.001451 > siesta: 9 -0.009523 0.004610 0.013162 > siesta: 10 0.019411 -0.008606 -0.009840 > siesta: 11 0.002485 -0.010131 0.005577 > siesta: 12 0.001815 -0.012882 -0.010601 > siesta: 13 0.002293 -0.027828 0.004317 > siesta: 14 0.003541 -0.004099 -0.003595 > siesta: 15 -0.004545 -0.005597 0.018501 > siesta: 16 -0.002375 0.005693 -0.016887 > siesta: 17 0.007410 -0.017539 -0.001118 > siesta: 18 0.015827 -0.012960 -0.005705 > siesta: 19 -0.010629 -0.007460 -0.001509 > siesta: 20 0.008748 0.000683 -0.004698 > siesta: 21 -0.004943 0.029703 -0.008064 > siesta: 22 -0.003202 0.004500 0.030359 > siesta: 23 -0.009982 -0.003884 -0.044988 > siesta: 24 0.012110 -0.008463 -0.054600 > siesta: 25 0.014891 0.000114 0.030375 > siesta: 26 0.011714 0.007854 -0.002981 > siesta: 27 -0.006905 -0.024651 0.005841 > siesta: 28 0.002399 -0.019318 0.013282 > siesta: 29 -0.019852 0.006114 -0.030653 > siesta: 30 -0.023687 0.004007 -0.028044 > siesta: 31 0.023195 -0.022556 -0.014991 > siesta: 32 0.024333 -0.033430 -0.028258 > siesta: 33 -0.014937 0.006378 0.009843 > siesta: 34 0.004912 -0.019004 -0.008074 > siesta: 35 0.006259 -0.012323 0.009612 > siesta: 36 -0.010453 0.017222 0.013853 > siesta: 37 0.001281 0.001284 -0.002971 > siesta: 38 0.009564 0.017869 -0.013170 > siesta: 39 0.004626 -0.016715 -0.008634 > siesta: 40 0.008447 -0.028871 -0.007756 > siesta: 41 -0.020042 -0.027721 -0.021694 > siesta: 42 0.020027 -0.019833 -0.095505 > siesta: ---------------------------------------- > siesta: Tot 0.016519 -0.011098 -0.032128 > > > -- > > Carlo Motta > * Ph.D. Student in Materials Science* > University Office Phone : +39 02 6448-5183 > Department of Materials Science > University of Milano-Bicocca > Via R. Cozzi 53, 20125 Milano, Italy >
