[QE-users] Unoccupied molecular orbitals near the vacuum level.

[Steven Best]

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'
 /

#######################################################################################

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.quantum-espresso.org/pipermail/users/attachments/20211221/c3c04242/attachment.html>