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 Question about Geometry Optimization Date: 2015/08/22 18:31 Name: Eike F. Schwier   Dear "MD.type RF" experts,I have come across a rather strange behaviour during one of my bulk unit-cell optimisations and would like to ask, if anybody can explain to my why this is occurring and if there are any parameters I may change / set / optimize to prevent this from happening.In my relaxation of the FRAC coordinates (RF) of TlBiSe2, I found that there was a lateral force on the Se atoms (not on Tl or Bi). If Se positions are allowed to relax laterally (MD.Fixed.XYZ 0 0 0) all Se move ~0.1 A away from their original positions until the convergence criterion is reached. This is due to a force Fx,Fy~0.008 Ha/bohr. If I the restart the optimisation from the "relaxed" coordinates, the Se atoms move back to (almost) their original positions before the first lateral relaxation. This seemed puzzling as I do not expect any more relaxation, if I restart the RF from its already relaxed coordinates. Btw: If I choose EF instead of RF I get the same behaviour.Se positions in Ang (original, first relax, second relax)1,x 1,y 2,x 2,y 3,x 3,y 0 0 0 0 2.12 1.22398 0.10537 0.04579 -0.10537 -0.04578 2.22532 1.26981 0.00031 0.00096 -0.00031 -0.00096 2.12024 1.22495 4,x 4,y 5,x 5,y 6,x 6,y2.12 1.22398 0 2.44797 0 2.447972.01463 1.17812 0.10537 2.49384 -0.10533 2.40213 2.11978 1.22314 0.00022 2.4488 -0.00024 2.447I tried to change the basisset, the cutoff and the k-mesh, but the initial force on the Se atoms remains nonzero regardless of my choice of parameters. However, if I displace the first Se atom (dX >= 0.+0.00001001 FRAC) the lateral forces Fx and Fy become zero (Fx,Fy~0.00005 Ha/bohr) If I choose a dX <= 0.00001 forces are non-zero again. My suspicion is, that I break some internal symmetry in the calculation with the Se displacement larger than 0.00001 FRAC. But I do not understand why a broken symmetry should change the forces to such an extend. Furthermore, if the groundstate is indeed one with the Se atoms displaced and the symmetry broken, then why do the Se atoms move back to the symmetry positions if I restart the calculation? Is the initial symmetry conserved in the RF (EF) run, even if the fractional coordinates are changed to break it?best regards,Eike## Definition of Atomic Species#Species.Number 3Atoms.Number 12Atoms.SpeciesAndCoordinates.Unit FRAC # Ang|AUAtoms.UnitVectors.Unit Ang # Ang|AU## SCF or Electronic System#scf.XcType GGA-PBE # LDA|LSDA-CA|LSDA-PW|GGA-PBEscf.ElectronicTemperature 300.0 # default=300 (K)scf.energycutoff 300.0 # default=150 (Ry)scf.maxIter 160 # default=40scf.EigenvalueSolver band # Recursion|Cluster|Bandscf.Kgrid 5 5 3 # means n1 x n2 x n3scf.Generation.Kpoint MP # regular|MPscf.Mixing.Type rmm-diisk # Simple|Rmm-Diis|Gr-Pulayscf.Init.Mixing.Weight 0.3 # default=0.30 scf.Min.Mixing.Weight 0.001 # default=0.001 scf.Max.Mixing.Weight 0.600 # default=0.40 scf.Mixing.History 20 # default=5scf.Mixing.StartPulay 10 # default=6scf.criterion 1e-9 # default=1.0e-6 (Hartree) scf.restart off## MD or Geometry Optimization#MD.Type RF #MD.maxIter 1 # default=1MD.Opt.criterion 1.0e-5scf.fixed.grid 0.00000000000000 0.00000000000000 0.00000000000000
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