[Pw_forum] Magnetization in the Adsorption of Molecular O2

Tue Jun 11 14:20:26 CEST 2013

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
> 
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