Dear 1009u, the things to look at are: 1. Your acoustic modes are close enough to zero, that means that, most probably, an insufficient MeshCutoff is not your problem. 2. The frequencies which are supposed to be degenerate are reasonably close, that means that numerical errors due to k-mesh etc., are not obviously to blame. 3. You say that not all frequencies are bad from which I conclude that the rest (out of 7, or so? I do not have the symmetry decomposition at hand) modes, are OK. This is not bad. For the modes which worry you, you should have a more attentive look at - the effect of the displacement magnitude; some modes are strongly anharmonic and depend on it.
If the difference persists, it can be either - the general DFT problem (do you have other calculations to compare?), or - the experiment problem (are the data reliable? Are other experiments available? Do these frequencies strongly depend on temperature?). After eliminating all these source of difference, whatever rests is a purely Siesta problem - pseudos and bases. It does not mean, though, that you can tune it easily. How good is your La ? Semicores etc. Do the elastic properties come out right? To test them might be not so easy for a hexagonal structure. Take a simple cubic compound with your La and check the bulk modulus. Compare it against WIEN2k or other all-electron calculation. Good luck Andrei Postnikov > Thank you Sir, > > I have done few calculations of zone center phonons using primitive unit. > > But i got wrong zone center phonon frequencies ( not all ) in LaB6 system. > Similarly in carbide systems. > > e.g. For LaB6 system > > Expt. freq > Freq (by SIESTA) > > F1u mode ( translation) 210 > 181.12,182.30, 182.97 > > F1u mode (bending) 1095 > 877.12, 877.60, 879.85 > > > and also > > Accoustical > modes 0.0310, > 0.0396, 0.0766, > > > Many times i changed the meshcutoff (kept more than 400 Ryd) and kgrid ( > kept more than 25 Bohr) to get the required frequiences, But only the > digits after decimal place changes i.e meshcutoff and kgrid doesn't > change > the frequencies appreciably. > > Is the problem because of Pseudo-potential and basis.? > > I also checked the frequencies using GGA approximation to particle density > functional, but no success? > > hope that the information given is sufficient. > > With Regards > > 1009u > > >
