Sophie, >> >> My calculation for 64-atoms:
Why do you have to go to 64 atoms to get this kind of quantities? The unit cell should suffice. Nevertheless: >> 1) Lattice Constant: I changed the lattice parameter and find the energy >> and plotting that I find 5.45 Ang, instead of 5.40 as reported in the >> paper. Why this so different? >> >> 2) Bulk modulus: With B = V*Curvature = V*2c3 = 15 MPa, which is far >> less >> than 98.6 MPa? Why? Here, c3 is defined as E =C1 + c2*V + c3*V^2, curve >> fit to the E vs V curve. This is my opinion based from previous personal experience, and doesn't mean that the explanation is completely correct. The more experienced users can correct me if I'm wrong. One of the possible reasons is that you have a combination of two factors that are leading you to some error there: using a 64 atom cell and using a quadratic equation as an equation of state. If you are using a quadratic equation, you have to be very sure that your total energy really behaves so, because your DFT calculations will show terms in the total energy that would be of a higher order - by choosing a quadratic dispersion, you are choosing to ignore these terms. Anyway, using a quadratic dispersion is never a very good procedure. In my experience, a cubic would be better because of the reasons stated above. For the supercell you are using, any small change in the lattice parameter will lead to rather large energy differences. So you might be missing a good description in the minimum of the curve, which is one factor that could be affecting your results - after all, the bulk modulus is calculated at the point of minimum energy. If you are not describing it well through your fit, then you will have poor results. A much better procedure would be to use the unit cell, and a Murnaghan fit to determine directly B, B' and V_0, the equilibrium volume. Using a small cell, the energy changes will be smaller with changes in the lattice parameters, in comparison to the 64-atom supercell. Moreover, the Murnaghan (or Birch-Murnaghan) equation of state can handle larger changes while giving you rather accurate results. As to the cohesive energy, I can't think of anything right now, that could be a source to the brutal discrepancies you are seeing. Maybe the k-point sampling is not so good but, being Si and a 64-atom supercell, I would not really guess that would be the problem. Best regards, Marcos >> >> 3) Cohesive Energy: I find energy per atom 107.759eV in the bulk. Now to >> calculate energy per atom I tried to use the suggestions given at >> http://www.mail-archive.com/[email protected]/msg03118.html >> I get -7.49eV/atom as pseudopotential calculation, and -576.38eV/atom as >> ae. So, which one is the energy of the free atom. None is a good one for >> comparing with the energy I got from bulk to find the cohesive energy. >> >> I really tried to search the archive and find solutions to these. I >> could >> not find any explicit answers. I am not sure what I am missing. *Some >> expert's simple directions can solve the problem right away*. I really >> need this help. >> >> >> Thanks in advance, >> Sophia >> Univ. of California - Berkeley >> >> Attached fdf file >> >> ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ >> >> # >> ----------------------------------------------------------------------------- >> # FDF for a cubic c-Si supercell with 64 atoms >> # >> # E. Artacho, April 1999 >> # >> ----------------------------------------------------------------------------- >> >> SystemName 64-atom Si >> SystemLabel Si >> >> NumberOfAtoms 64 >> NumberOfSpecies 1 >> >> %block ChemicalSpeciesLabel >> 1 14 Si >> %endblock ChemicalSpeciesLabel >> >> PAO.BasisSize DZP >> PAO.EnergyShift 20 meV >> >> %Block PAO.Basis >> Si 3 -0.46385 >> n=3 0 2 E 15.42551 4.96988 >> 7.00000 4.37722 >> 1.00000 1.00000 >> n=3 1 2 E 4.69636 3.83128 >> 7.00000 4.09123 >> 1.00000 1.00000 >> n=3 2 1 E 11.96912 0.03131 >> 4.55426 >> 1.00000 >> %EndBlock PAO.Basis >> >> >> LatticeConstant 5.430 Ang >> %block LatticeVectors >> 2.000 0.000 0.000 >> 0.000 2.000 0.000 >> 0.000 0.000 2.000 >> %endblock LatticeVectors >> >> %block kgrid_Monkhorst_Pack >> 2 0 0 0.0 >> 0 2 0 0.0 >> 0 0 2 0.0 >> %endblock kgrid_Monkhorst_Pack >> >> >> MeshCutoff 40.0 Ry >> >> MaxSCFIterations 100 >> DM.MixingWeight 0.3 >> DM.NumberPulay 3 >> DM.Tolerance 1.d-3 >> DM.UseSaveDM >> XC.functional LDA >> XC.authors CA >> >> >> SolutionMethod diagon >> ElectronicTemperature 25 meV >> >> WriteForces true >> WriteCoorStep true >> >> MD.TypeOfRun cg >> MD.NumCGsteps 0 >> MD.MaxCGDispl 0.1 Ang >> MD.MaxForceTol 0.01 eV/Ang # earler 0.04 >> SaveRho true >> >> AtomicCoordinatesFormat ScaledCartesian >> %block AtomicCoordinatesAndAtomicSpecies >> 0.00000 0.00000 0.00000 1 >> 0.00000 0.50000 0.50000 1 >> 0.25000 0.25000 0.75000 1 >> 0.25000 0.75000 0.25000 1 >> 0.50000 0.00000 0.50000 1 >> 0.50000 0.50000 0.00000 1 >> 0.75000 0.25000 0.25000 1 >> 0.75000 0.75000 0.75000 1 >> 0.00000 0.00000 1.00000 1 >> 0.00000 0.50000 1.50000 1 >> 0.25000 0.25000 1.75000 1 >> 0.25000 0.75000 1.25000 1 >> 0.50000 0.00000 1.50000 1 >> 0.50000 0.50000 1.00000 1 >> 0.75000 0.25000 1.25000 1 >> 0.75000 0.75000 1.75000 1 >> 0.00000 1.00000 0.00000 1 >> 0.00000 1.50000 0.50000 1 >> 0.25000 1.25000 0.75000 1 >> 0.25000 1.75000 0.25000 1 >> 0.50000 1.00000 0.50000 1 >> 0.50000 1.50000 0.00000 1 >> 0.75000 1.25000 0.25000 1 >> 0.75000 1.75000 0.75000 1 >> 0.00000 1.00000 1.00000 1 >> 0.00000 1.50000 1.50000 1 >> 0.25000 1.25000 1.75000 1 >> 0.25000 1.75000 1.25000 1 >> 0.50000 1.00000 1.50000 1 >> 0.50000 1.50000 1.00000 1 >> 0.75000 1.25000 1.25000 1 >> 0.75000 1.75000 1.75000 1 >> 1.00000 0.00000 0.00000 1 >> 1.00000 0.50000 0.50000 1 >> 1.25000 0.25000 0.75000 1 >> 1.25000 0.75000 0.25000 1 >> 1.50000 0.00000 0.50000 1 >> 1.50000 0.50000 0.00000 1 >> 1.75000 0.25000 0.25000 1 >> 1.75000 0.75000 0.75000 1 >> 1.00000 0.00000 1.00000 1 >> 1.00000 0.50000 1.50000 1 >> 1.25000 0.25000 1.75000 1 >> 1.25000 0.75000 1.25000 1 >> 1.50000 0.00000 1.50000 1 >> 1.50000 0.50000 1.00000 1 >> 1.75000 0.25000 1.25000 1 >> 1.75000 0.75000 1.75000 1 >> 1.00000 1.00000 0.00000 1 >> 1.00000 1.50000 0.50000 1 >> 1.25000 1.25000 0.75000 1 >> 1.25000 1.75000 0.25000 1 >> 1.50000 1.00000 0.50000 1 >> 1.50000 1.50000 0.00000 1 >> 1.75000 1.25000 0.25000 1 >> 1.75000 1.75000 0.75000 1 >> 1.00000 1.00000 1.00000 1 >> 1.00000 1.50000 1.50000 1 >> 1.25000 1.25000 1.75000 1 >> 1.25000 1.75000 1.25000 1 >> 1.50000 1.00000 1.50000 1 >> 1.50000 1.50000 1.00000 1 >> 1.75000 1.25000 1.25000 1 >> 1.75000 1.75000 1.75000 1 >> %endblock AtomicCoordinatesAndAtomicSpecies >> > -- Dr. Marcos Verissimo Alves Post-Doctoral Fellow Unité de Physico-Chimie et de Physique des Matériaux (PCPM) Université Catholique de Louvain 1 Place Croix du Sud, B-1348 Louvain-la-Neuve Belgique ------ Gort, Klaatu barada nikto. Klaatu barada nikto. Klaatu barada nikto.
