Dear Vic I would say: keep in mind that a molecule interacting with a surface is not anymore an isolated molecule... But rather than a long explanation about delocalisation and static correlation errors in DFT, you might find useful to flick through these nice papers:
Insights into Current Limitations of Density Functional Theory Aron J. Cohen et al. Science 321, 792 (2008) Localization and Delocalization Errors in Density Functional Theory and Implications for Band-Gap Prediction Paula Mori-Sanchez et al. Phys Rev Lett 100, 146401 (2008) Discontinuous Nature of the Exchange-Correlation Functional in Strongly Correlated Systems Paula Mori-Sanchez et al. Phys Rev Lett 102, 066403 (2009) HTH Giuseppe On Tuesday 11 June 2013 12:59:23 Vic Bermudez wrote: > Hello, > > I'm getting what seem to be very strange results when trying to model > the > adsorption of molecular O2 on a semiconductor surface (GaSb). To begin, > everything looks reasonable for free O2, modeled as a cubic lattice of O2 > spaced 20 Angstroms apart. The triplet ground state lies 1.08 eV below the > lowest singlet state, in reasonable agreement with the experimental result > of 0.98 eV. > Now when I place a single O2 molecule near a GaSb surface the > preliminary > result is that a "superoxo-like" Ga-O-O species forms during relaxation > with one O bonded to Ga and the other left dangling. With > starting_magnetization(1)=0.17 for each O the adsorption energy is 0.52 eV > with > total magnetization = -1.30 Bohr mag/cell > absolute magnetization = 1.42 Bohr mag/cell > > If instead I force a triplet by using Tot_Magnetization=2.0 I get a > slightly smaller adsorption energy of 0.49 eV (i.e., a less stable > structure). If I force a singlet (Tot_Magnetization=0.0) I get an even > smaller adsorption energy, 0.43 eV. Adsorption energy is defined here as > E(free O2) + E(bare surface) - E(O2+surface) > I've tried letting the 2 O atoms be magnetically inequivalent, i.e., > using > an O1 and an O2 with the same pseudopotential but with > starting_magnetization(1)=0.27, > starting_magnetization(2)=0.07 > Atom #1 is the dangling O, which I would expect to have a higher density of > unpaired spin. However, this converges to essentially the same results for > total and absolute magnetization as before. > Clearly I'm doing something wrong here, since I expect the total spin to > be conserved in adsorption on a diamagnetic surface. The bare-surface > reconstruction I'm studying has all dangling bonds either empty or doubly > occupied and hence has no magnetic properties of its own. This doesn't > seem to be a typical case of spin contamination. From what I've read in > the literature, spin contamination in DFT is usually smaller than in UHF > and also involves the admixing of higher spin states. Hence, a > spin-contaminated triplet would have a total magnetization of >2.0 and not > the 1.3 that I'm getting. > If it's of use I'm including the input for the run with two different O > spins as a sample. A few of comments may be in order. To save space in this > message I've removed most of the atom positions. The two-dimensional slab > has a dipole moment so a correction dipole is included in the vacuum space. > "nstep" and "electron_maxstep" are set absurdly high to avoid a problem > I've been having with "maximum number of steps exceeded" after only 60 or > so optimizations steps (a different problem, not the subject of this > message). The H5 pseudoatoms are for terminating the unsaturated Ga atoms > at the "bottom" of the slab. > > &CONTROL > calculation='relax', > restart_mode='from_scratch', > title='GaSb alpha-4x3 + O2; start with #151846 opt. pos.; make O2 > asymmetric', > pseudo_dir="/work1/app/espresso/pseudo", > outdir='/work2/bermudvm/GaSb+O2__outdir', > etot_conv_thr=1.0D-5, > forc_conv_thr=5.0D-4, > nstep=10000, > wf_collect=.TRUE., > verbosity='default', > tefield=.TRUE., > dipfield=.TRUE. > / > > &SYSTEM > ibrav=8, > a=17.1119841047, b=12.8339880785, c=44.0, > nat=130, > ntyp=5, > ecutwfc=30.0, > ecutrho=300.0, > london=.TRUE., > occupations='smearing', > smearing='gaussian', > degauss=0.005D0, > nspin=2, > starting_magnetization(3)=0.27, > starting_magnetization(4)=0.07, > edir=3, > eamp=0.0, > emaxpos=0.5, > eopreg=0.1, > input_dft='pw91' > / > > &ELECTRONS > electron_maxstep=3000, > conv_thr=1.0D-7, > mixing_mode='local-TF' > / > > &IONS > ion_dynamics='bfgs', > upscale=10000.D0 > / > > ATOMIC_SPECIES > Ga 69.72 Ga.pw91-nsp-van.UPF > Sb 121.75 Sb.pw91-n-van.UPF > O1 15.9994 O.pw91-van_ak.UPF > O2 15.9994 O.pw91-van_ak.UPF > H5 1.25 H.pz-vbc_125.UPF > > ATOMIC_POSITIONS angstrom > O1 -1.057077087 -0.134186302 8.773421904 > O2 -2.125029769 0.459941835 8.302768681 > Sb 2.139040375 4.993070914 6.782713316 > Ga 2.132538190 -5.199966759 5.947206011 > Ga 6.277129268 1.147167971 5.989478016 > . > . > . > . > H5 -7.635543780 -4.277997026 -6.285642281 0 0 0 > H5 -3.357548712 -0.000000728 -6.285641379 0 0 0 > H5 -3.357548071 4.277997544 -6.285641893 0 0 0 > H5 -3.357547485 -4.277997519 -6.285642688 0 0 0 > > K_POINTS automatic > 3 4 1 0 0 0 > > I would greatly appreciate any advice regarding this situation. > > Thank You, > Vic Bermudez > > Victor M. Bermudez > Code 6876 > U.S. Naval Research Laboratory > 4555 Overlook Ave., S.W. > Washington, DC 20375-5347 > > Phone: 202-767-6728 > FAX: 202-767-1165 > E-mail: victor.bermudez at nrl.navy.mil > > _______________________________________________ > Pw_forum mailing list > Pw_forum at pwscf.org > http://pwscf.org/mailman/listinfo/pw_forum ******************************************************** - Article premier - Les hommes naissent et demeurent libres et ?gaux en droits. Les distinctions sociales ne peuvent ?tre fond?es que sur l'utilit? commune - Article 2 - Le but de toute association politique est la conservation des droits naturels et imprescriptibles de l'homme. Ces droits sont la libert?, la propri?t?, la s?ret? et la r?sistance ? l'oppression. ******************************************************** Giuseppe Mattioli CNR - ISTITUTO DI STRUTTURA DELLA MATERIA v. Salaria Km 29,300 - C.P. 10 I 00015 - Monterotondo Stazione (RM) Tel + 39 06 90672836 - Fax +39 06 90672316 E-mail: <giuseppe.mattioli at ism.cnr.it> ResearcherID: F-6308-2012
