Dear Ziba,it seems that you are randomly trying different options.Try to be more systematic.MeshCutoff of 870 Ry seems to be an overkill -but certainly not an error; if your tests show that you need as much as that, so be it.Did you have a look at your structures with visualisation tools - is everything as it should be, no obvious structure error?If your imaginary frequency is genuine and reproducible,I'd look at the eigenvector of this mode, give a small displacementto atoms along this eigenvector, and make the relaxation again. The total energy ought to be lower than in your initial starting point;this is the idea of an imaginary frequency: you displace the atomswithin the corresponding mode, and the energy goes down.Hence you were not at the equilibrium; try better.No way around.Best regardsAndrei Postnikov ----- Ziba Torkashvand <zi.torkashv...@gmail.com> a écrit : > Thank you Professor Postnikov,Here I will provide two examples of my workI have a hexagonal structure containing 18 atoms1. I have substituted one atom with a gaseous one and relaxed it up to 0.000794 eV/A and 0.00458798 kBarthen I calculated the phonon and fortunately, there is no negative frequency while the force for the first step is approximated as 0.008174 eV/A 2. I have substituted two atoms with two gaseous ones and relaxed it up to 0.000549 eV/A and 0.00290065 kBarthen I calculated the phonon and unfortunately, there are frequencies as -206.30292747012965 while the force for the first step is approximated as 0.015862 eV/A For both cases, I have used same inputs asFor relaxation:MeshCutoff 870 Ry DM.MixingWeight 0.02 DM.NumberPulay 3 %block kgrid_Monkhorst_Pack 6 0 0 0.0 0 6 0 0.0 0 0 1 0.0 %endblock kgrid_Monkhorst_Pack
For phonon: MeshCutoff 750 Ry DM.MixingWeight 0.02 DM.NumberPulay 8 %block kgrid_Monkhorst_Pack 3 0 0 0.0 0 3 0 0.0 0 0 1 0.0 %endblock kgrid_Monkhorst_Pack I repeated phonon calculation for the second case with %block kgrid_Monkhorst_Pack 6 0 0 0.0 0 6 0 0.0 0 0 1 0.0 %endblock kgrid_Monkhorst_Pack and the resulting force for the first step is 0.013928 eV/A I'm confused about what is happening in these two similar examples.
-- SIESTA is supported by the Spanish Research Agency (AEI) and by the European H2020 MaX Centre of Excellence (http://www.max-centre.eu/)
