Dear Andrei, So how do I calculate the total energy of a single atom with Siesta?I should use the same pseudopotential as what I use for the molecule, right?But for a single atom,do I place that atom at the origin? Thank you very much. Best regards, Haoliang Chen
在2010-04-25 14:30:07，[email protected] 写道： >Dear Haoliang, >I think when you compare total energies from Siesta and from Atom, >you introduce an unknown error, because they are calculated >in quite different way (mesh, basis issues appear in Siesta >but not in Atom). It is better to compare comparable values, >i.e. total energy of dimer against that of two distant atoms. >Define a large enough unit cell to accommodate two separated atoms >and, keeping the cell fixed, move the atoms together >and trace the energy; hopefully you'll find a minimum at >some distance. >In doing this, do not forget to make the calculation magnetic >and to set the magnetic moments on isolated atoms. > >Best regards > >Andrei Postnikov > > > >> Dear all, >> I was trying to calculate the binding energy of a SiC molecule.The formula >> I use is : E(SiC)-E(Si)-E(C), and then divide it by the number of total >> atoms to get the binding energy per atom. The experimental value is >> 2.34eV, and what I got is over 3.44eV which is way off the experimental >> value. I used the ATOM program to calculate the atomic energy of Si and C >> which might not be a good idea. I got the energy of Si atom and C atom >> from all-electron and pseudopotential calculation separately. The energy >> of Si is -7.5 Ry and the energy of C is -10.69 Ry using pseudopotential >> calculation. Then I calculated the total energy of a SiC molecule with >> structural relaxation which is not satisfying. The total energy is >> -254.267753eV which is equal to -18.696 Ry approximately. That gives me >> the binding energy per atom 3.44eV. >> I used the pseudopotential of Si from ATOM program of Siesta and the >> pseudopotential from Javier Junquera (Flavour of the pseudopotential: >> Troullier-Martins. Exchange and correlation functional: LDA >> (Ceperley-Alder). Relativistic: no. Core corrections: no. Valence >> reference configuration: 2s2 2p2 3d0 4f0 Cutoff radius: 2s 1.25 bohr 2p >> 1.25 bohr 3d 1.25 bohr 4f 1.25 bohr ). >> This is my input file: >> SystemName SiC molecule >> SystemLabel SiC >> NumberOfAtoms 2 >> NumberOfSpecies 2 >> MeshCutoff 50 Ry >> %block ChemicalSpeciesLabel >> 1 14 Si # Species index, atomic number, species label >> 2 6 C >> %endblock ChemicalSpeciesLabel >> AtomicCoordinatesFormat Ang >> %block AtomicCoordinatesAndAtomicSpecies >> 0.000 0.000 0.000 1 >> 1.870 0.000 0.000 2 >> %endblock AtomicCoordinatesAndAtomicSpecies >> %block Ps.lmax >> C 1 >> %endblock Ps.lmax >> >> SpinPolarized T >> DM.MixingWeight 0.01 >> MeshCutoff 150 Ry >> MD.TypeOfRun Broyden >> MD.VariableCell F >> MD.NumCGsteps 100 >> >> PAO.BasisSize DZ >> XC.functional GGA >> XC.authors PBE >> MaxSCFIterations 400 >> SCFMustConverge F >> >> save-rho T >> save-delta-rho T >> save-total-potential T >> save-neutral-atom-potential T >> save-hs T >> LongOutput T >> >> I also attached the output file of Si and C atom. So what I'm wondering is >> why the binding energy is not right. Is it because of the pseudopotential >> I use or the generation method of pseudopotential? Or something else like >> basis size, DM.Mixing weight etc? And how to get a successful optimization >> of geometry? Thanks >> >> Yours, >> Haoliang >> >
