I have used PWSCF v6.3 and optimized the geometry of an isolated H2 molecule in
a 24 Angstrom box.
I have then used pp.x to plot the occupied and un-occupied molecular orbitals.
With nbnd=2 chosen for the scf calculation I obtain energy
bands of:
-10.3337 0.1108 eV
occupation numbers
1.0000 0.0000
With nbnd=4 chosen I obtain energy bands of:
-10.3343 -0.0797 0.1110 0.1375 eV
occupation numbers
1.0000 0.0000 0.0000 0.0000
With nbnd=2 the plots of the occupied and unoccupied molecular orbitals look
like the expected bonding and anti-bonding
H2 molecular orbitals. However, if nbnd=4 is chosen, an unoccupied orbital
(band 2) with a shape consisting of a number of shells is
located below the anti-bonding unoccupied orbital (band 3). And another orbital
with a similar shell structure (band 4) is located
above the anti-bonding orbital which appears to have the opposite phase to band
2.
I have calculated the vacuum level energy far away from the molecule to be
approximately 0.003 eV by plotting the average potential.
I think that the shape of the orbitals for nbnd=4 is possibly a result of a
limitation in the plane wave method that occurs for unoccupied orbitals
with an energy greater than or close to the vacuum level? Can anybody provide
some insight into this behaviour?
Kind regards,
Steven Best
PhD Student,
School of Chemistry and Physics,
Queensland University of Technology, Brisbane, Australia
Below are my script files.
#######################################################################################
# H2 molecule with nbnd = 2.
&CONTROL
calculation = 'scf',
verbosity = 'high',
restart_mode = 'from_scratch',
wf_collect = .T.,
nstep = 200,
tstress = .true.,
tprnfor = .true.,
outdir = './',
prefix = 'SCF_H2_molecule_nbnd_2_t007',
etot_conv_thr = 1.0d-6,
forc_conv_thr = 1.0d-5,
pseudo_dir = './pseudo'
/
&SYSTEM
ibrav = 1, A = 24.0,
nat = 2, ntyp = 1,
nbnd = 2,
ecutwfc = 60.0, ecutrho = 480.0
/
&ELECTRONS
electron_maxstep = 100,
conv_thr = 1.0d-8,
mixing_mode = 'plain',
mixing_beta = 0.4, mixing_ndim = 8,
diagonalization = 'david'
/
ATOMIC_SPECIES
H 1.008 H.pbe-kjpaw_psl.1.0.0.UPF
ATOMIC_POSITIONS (angstrom)
H 12.00 12.00 12.00 0 0 0
H 12.750482666 12.00 12.00
K_POINTS (gamma)
#######################################################################################
# Post processing for H2 molecular orbitals.
# Number of bands = 2.
&INPUTPP
prefix = 'SCF_H2_molecule_nbnd_2_t007',
outdir = './',
filplot = 'H2_molecule_nbnd_2_wfc_t007',
plot_num = 7,
kpoint(1) = 1,
kband(1) = 1,
kband(2) = 2,
lsign=.true.
/
&PLOT
nfile = 1,
filepp(1) = 'H2_molecule_nbnd_2_wfc_t007', weight(1)=1.0,
iflag = 3,
output_format = 6,
fileout = '.cube'
/
#######################################################################################
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