Giacomo, I see you are using a mesh cutoff of 100 Ry for your calculation. This could be too small by itself for O, which is a hard pseudo in comparison with other elements. I believe converged calculations for compound with O should use at least 180 Ry (an equivalent of 45 Ry in plane waves), which is not the most converged, either. 200 Ry is roughly equivalent to a PW 25 Ry, and this would be good only for ultrasoft pseudos, not norm-conserving ones.
But then there is the problem of the eggbox effect. Probably with 100 Ry the grid is so coarse you are having spurious numerical problems close to the energy minimum. You can go to a higher cutoff to minimize this problem, and this could even prove advantageous because your SCF will converge in less steps, and your calculation will require less CG moves to get to the minimum. Another "trick" is to make MaxCGDisplacement smaller than the default. This helps locate the energy minimum more easily if the minimum is shallow. I don't know about the reasons why it is difficult to reproduce the energetics of TiO2 phases, but if this is so, you might also want to go to higher k-samplings. Are your results for the 4x4x1 grid consistent with what you see in literature? Cheers, Marcos On Thu, Jan 20, 2011 at 2:23 AM, Giacomo Giorgi <[email protected]> wrote: > Dear All, > I am nowadays performing some benchmark calculations about bulk TiO2 > (rutile and anatase). The idea is to do some calculations on different > surface reconstructions. > I think that (I am almost sure) a pseudo for Ti with p orbitals in the > valence is almost mandatory for getting reliable results. > > The problem I am facing is the following. I found a pseudo LDA for Ti > in the website of Siesta together with the optimized basis set (Ti semicore > states (3s and 3p) in valence) > > For O I use the pseudo I found in the website (Oxygen tm2, distrib with > SIESTA, and a DZP basis set) > > When I test the structure of rutile (MD.Variable.Cell false), it is very > difficult for me to get the convergence (more than 1000 CG steps). It seems > impossible indeed to reduce the forces below 0.03 eV/Ang. This seems to me > quite strange, also because I am restarting from a 4x4x1 Gamma centered > previously converged structure. > It is impossible that increasing the sampling from 4x4x1 to 4x4x2 the > number of CG changes from 11 to more than 1000..... > I thus think that if convergence is not achieved for this small system > (rutile, only 2 Ti atoms) even more difficult (if not impossible) will be to > perform calculations on slabs. > > > Thus I wonder if someone can give me some suggestions in order to fix this > problem. Is it a pseudo problem? Is it sufficient a 4e- pseudo in the > valence for optimally describing TiO2 properties (I am quite doubtful about > it)? I already know form literature that it is very difficult to reproduce > the experimental energetic ordering of anatase and rutile. > > In case, has anyone a pseudo tested for Ti (PBE even better) used for TiO2 > based systems to share with me? > > SystemName TiO2_rutilo > SystemLabel TiO2_rutilo > NumberOfAtoms 6 > NumberOfSpecies 2 > %block ChemicalSpeciesLabel > 1 22 Ti > 2 8 O > %endblock ChemicalSpeciesLabel > %block PAO.Basis # Define Basis set > Ti 5 1.91 > n=3 0 1 E 93.95 5.20 > 5.69946662616249 > 1.00000000000000 > n=3 1 1 E 95.47 5.20 > 5.69941339465994 > 1.00000000000000 > n=4 0 2 E 96.47 5.60 > 6.09996398975307 5.09944363262274 > 1.00000000000000 1.00000000000000 > n=3 2 2 E 46.05 4.95 > 5.94327035784617 4.70009988294302 > 1.00000000000000 1.00000000000000 > n=4 1 1 E 0.50 1.77 > 3.05365979938936 > 1.00000000000000 > %endblock PAO.Basis > > %block PAO.BasisSizes > O DZP > %endblock PAO.BasisSizes > > XC.functional LDA > XC.authors CA > LatticeConstant 4.59220 Ang > %block LatticeVectors > 1.0000 0.000 0.000 > 0.0000 1.000 0.000 > 0.0000 0.000 .64435346892556944384 > %endblock LatticeVectors > > %block kgrid_Monkhorst_Pack > 4 0 0 0.5 > 0 4 0 0.5 > 0 0 2 0.5 > %endblock Kgrid_Monkhorst_Pack > > MeshCutoff 100.0 Ry > > MaxSCFIterations 1000 > DM.MixingWeight 0.05 > DM.NumberPulay 3 > DM.Tolerance 1.d-4 > > SolutionMethod diagon > > SpinPolarized true > LongOutput true > > MD.TypeOfRun cg > MD.NumCGsteps 1000 > MD.VariableCell false #true > MD.MaxForceTol 0.01 eV/Ang #0.005 eV/Ang > WriteForces true > WriteCoorCerius true > WriteCoorXmol false > WriteDenchar true > WriteMullikenPop 1 > > > UseSaveData true > > Diag.ParallelOverK true > > AtomicCoordinatesFormat Fractional > %block AtomicCoordinatesAndAtomicSpecies > 0.00000 0.00000 0.00000 1 > 0.50000 0.50000 0.50000 1 > 0.30510 0.30510 0.00000 2 > 0.19490 0.80510 0.50000 2 > 0.69490 0.69490 0.00000 2 > 0.80510 0.19490 0.50000 2 > %endblock AtomicCoordinatesAndAtomicSpecies > > > > ..... > siesta: ============================== > Begin CG move = 1000 > ============================== > > outcoor: Atomic coordinates (fractional): > -0.00002412 0.01663451 -0.00906965 1 Ti 1 > 0.49999230 0.51667694 0.49093035 1 Ti 2 > 0.29977119 0.33204697 -0.00156798 2 O 3 > 0.20023279 0.83205355 0.49848870 2 O 4 > 0.70023510 0.70128165 -0.00155817 2 O 5 > 0.79979393 0.20130016 0.49847493 2 O 6 > > superc: Internal auxiliary supercell: 4 x 4 x 7 = 112 > superc: Number of atoms, orbitals, and projectors: 672 10080 11648 > > outcell: Unit cell vectors (Ang): > 4.592200 0.000000 0.000000 > 0.000000 4.592200 0.000000 > 0.000000 0.000000 2.959000 > > outcell: Cell vector modules (Ang) : 4.592200 4.592200 2.959000 > outcell: Cell angles (23,13,12) (deg): 90.0000 90.0000 90.0000 > outcell: Cell volume (Ang**3) : 62.4003 > > siesta: iscf Eharris(eV) E_KS(eV) FreeEng(eV) dDmax Ef(eV) > siesta: 1 -4921.3185 -4921.3185 -4921.3185 0.0000 -4.1784 > > siesta: E_KS(eV) = -4921.3185 > > siesta: Atomic forces (eV/Ang): > 1 0.001276 0.003726 -0.037969 > 2 -0.000729 -0.003555 -0.039057 > 3 0.003563 0.007823 0.004704 > 4 -0.006634 0.009005 -0.003266 > 5 -0.003276 -0.006490 0.003888 > 6 0.000690 -0.007992 0.002003 > ---------------------------------------- > Tot -0.005110 0.002516 -0.069698 > ---------------------------------------- > Max 0.039057 > Res 0.013671 sqrt( Sum f_i^2 / 3N ) > ---------------------------------------- > Max 0.039057 constrained > > > Thanks in advance! > Best > > Giacomo > > > > > > > * * * * * * * * * * * * * * * * * * * * * * * * * > * Giacomo Giorgi * > * Ph.D. * > * Dipartimento di Chimica e Istituto di * > * Scienze e Tecnologie Molecolari ISTM-CNR * > * Universita' degli Studi di Perugia * > * Via Elce di Sotto 8 06123 Perugia Italia * > * Tel: +39 075-5855618 Fax: +39 075-5855606 * > * E-mail: giac_at_thch.unipg.it * > * * * * * * * * * * * * * * * * * * * * * * * * * > > > > > > > "Oltre le illusioni di Timbuctu' > e le gambe lunghe di Babalu' c'era questa strada... > Questa strada zitta che vola via come una farfalla, > una nostalgia, nostalgia al gusto di curacao... > Forse un giorno meglio mi spieghero'..." > > (Paolo Conte, "Hemingway") > >
