[Pw_forum] Problem With Spin-Unrestricted Calculation for Adsorbed Singlet O2

Vic Bermudez victor.bermudez at nrl.navy.mil
Sun Sep 29 00:38:29 CEST 2013


Hello All,

	I am looking at the adsorption of molecular O2 on a GaSb surface. My
approach is first to relax with a spin-restricted calculation until the
changes in energy and forces begin to slow down. I then restart ("from
scratch") using this partially-relaxed structure and spin-unrestricted
calculations with Total_Magnetization = 0.0 or 2.0. The idea is to save a
little time vs. starting both of the unrestricted runs from a rough-guess
structure.
	Total_Magnetization = 2.0 corresponds to the spin-triplet ground state of
free O2. Total_Magnetization = 0.0 corresponds to the lowest excited state
of O2, which is an open-shell singlet. For the free molecule, locking the
total magnetization at 0.0 avoids spin contamination. This seems to be a
standard approach in the literature. Failure to do so results in a lower
total energy (for free O2) but a highly-contaminated solution. I'm assuming
(again with support from the literature) that the same approach is
applicable in the case of adsorption. In almost all the structures I've
looked at, the singlet state of adsorbed O2 is lower in energy than the
triplet, in agreement with results for O2 on other substrates. I need to
check this further, but I assume that this results from the redistribution
of charge density when the adsorption bond(s) form.
	With all that as background, here is my question. At the end of a converged
unrestricted calculation for Total_Magnetization = 0.0 I get something like
the following:

lsda relaxation :  a final configuration with zero
                        absolute magnetization has been found

     the program is checking if it is really the minimum energy structure
     by performing a new scf iteration without any "electronic" history

     The initial density is read from file :
     /work2/bermudvm/GaSb+O2-s13/tmp/pwscf.save/charge-density.dat


     negative rho (up, down):  0.183E-01 0.183E-01

     Adding external electric field

     Computed dipole along edir(3) :
        Dipole                0.6940 Ry au,          1.7640 Debye
        Dipole field          0.0001 Ry au

        Potential amp.       -0.0200 Ry
        Total length         74.8332 bohr


     Adding external electric field

     Computed dipole along edir(3) :
        Dipole                0.6940 Ry au,          1.7640 Debye
        Dipole field          0.0001 Ry au

        Potential amp.       -0.0200 Ry
        Total length         74.8332 bohr

     Starting wfc from file

     Writing output data file pwscf.save
     Check: negative starting charge=(component1):   -0.102736
     NEW-OLD atomic charge density approx. for the potential

     Check: negative/imaginary core charge=   -0.000001    0.000000
     Check: negative starting charge=(component1):   -0.102736

     negative rho (up, down):  0.183E-01 0.183E-01

     Adding external electric field

     Computed dipole along edir(3) :
        Dipole                0.6940 Ry au,          1.7640 Debye
        Dipole field          0.0001 Ry au

        Potential amp.       -0.0200 Ry
        Total length         74.8332 bohr


     Adding external electric field

     Computed dipole along edir(3) :
        Dipole                0.6940 Ry au,          1.7640 Debye
        Dipole field          0.0001 Ry au

        Potential amp.       -0.0200 Ry
        Total length         74.8332 bohr


     total cpu time spent up to now is   460045.7 secs

     per-process dynamical memory:  1832.4 Mb

     Self-consistent Calculation

     iteration #  1     ecut=    30.00 Ry     beta=0.70
     Davidson diagonalization with overlap
     ethr =  1.00E-06,  avg # of iterations =  1.0

     Threshold (ethr) on eigenvalues was too large:
     Diagonalizing with lowered threshold

     Davidson diagonalization with overlap

 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%
     from c_bands : error #         1
     too many bands are not converged
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%%%

     stopping ...

	This error message is repeated several times until the run eventually
aborts. My intuition is that I should just ignore this since what is
probably happening is that the program is trying to reach the lower-energy,
spin-contaminated state but can't because the system is locked at S = 0. Is
this correct, or is there a real problem here ?

	If it will help, here is the input (with most of the atomic coordinates
deleted) for the run that gave the above output:

&CONTROL
calculation='relax',
restart_mode='restart',
title='move O2 dimer #4 backbond; unrestricted S = 0; use final #91100
structure',
pseudo_dir="/work1/app/espresso/pseudo",
outdir='/work2/bermudvm/GaSb+O2-s13/tmp',
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=4,
ecutwfc=30.0,
ecutrho=300.0,
london=.TRUE.,
occupations='smearing',
smearing='gaussian',
degauss=0.002D0,
nspin=2,
tot_magnetization=0.0,
edir=3,
eamp=0.0,
emaxpos=0.5,
eopreg=0.1,
input_dft='pw91'
/

&ELECTRONS
electron_maxstep=2000,
conv_thr=1.0D-9,
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
O    15.9994 O.pw91-van_ak.UPF
H5    1.25   H.pz-vbc_125.UPF

ATOMIC_POSITIONS angstrom
O      -14.139100659   8.740950064   7.154491256
O      -12.720342683   8.304609872   7.300817121
Sb       2.096288690   4.750078727   6.845581956
Ga       1.894355025  -5.388868218   6.360351760
.
.
.
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. Sorry that this message is so
verbose, but I wanted to include the details.

Best Wishes,
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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