Dear SIESTA community: I have a question regarding the structural optimization using conjugate gradient (CG) method. Please refer to the following link for a jpg file.
http://cid-9a67d79e72d7a93d.photos.live.com/self.aspx/myfile/Energy-Force-CG.jpg As you can see from the above jpg file, I show the maximum atomic force and energy as a function of CG steps in process of structural optimization. Usually, we expect energy goes down with CG steps, in the meantime the maximum force decreases. However, we did not see this happens in our case. The force flucturates a lot around 0.25 eV/Ang, (we use 0.04 eV/Ang as optimization standard) and more surprisingly, the energy goes down and climbs up a big "hill" about 5 eV. This is difficult for us to understand. Our calculation was to study hole doping effect, we did succeed in optimizing our structure with "NetCharge" below 5 electrons. But when the "NetCharge" was increased to 8, the optimization becomes elusive. Our question is (1) How to understand such behaviour (in our picture above) in CG method? (2) Is there any other (better) way to reach structural optimization if "NetCharge" is nonzero and "SlabDipoleCorrection" is issued? Thanks alot! HHG Here is our input file: ===========start of input.fdf==== ============ SystemName Si-buck-surf SystemLabel Si-buck-surf NumberOfAtoms 56 Diag.ParallelOverK T SolutionMethod diagon NumberOfSpecies 2 ###################################### NetCharge 8 SlabDipoleCorrection .true. SimulateDoping .true. ###################################### %block ChemicalSpeciesLabel 1 14 Si 2 1 H %endblock ChemicalSpeciesLabel LatticeConstant 10.858 Ang %block LatticeVectors 1.000 0.000 0.000 0.000 1.000 0.000 0.000 0.000 10.000 %endblock LatticeVectors XC_functional LDA XC_authors CA %block PS.lmax Si 2 %endblock PS.lmax %block kgrid_Monkhorst_Pack 6 0 0 0.0 0 6 0 0.0 0 0 1 0.0 %endblock kgrid_Monkhorst_Pack PAO.EnergyShift 50. meV MeshCutoff 200. Ry DM.UseSaveDM F MaxSCFiterations 200 DM.MixingWeight 0.10 DM.NumberPulay 3 MD.NumCGsteps 200 MD.TypeOfRun CG MD.UseSaveXV F MD.UseSaveCG F SaveDeltaRho F SaveRho F AtomicCoordinatesFormat Ang %block AtomicCoordinatesAndAtomicSpecies 6.0665 10.2920 -5.3505 1 #Si -2.1063 2.4262 -5.0860 1 #Si -0.8249 -0.8868 0.8493 2 #H ...... ===========end of input.fdf================
