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

[Steven Best]

Dear Lorenzo,

Thanks for your reply.
I had not considered the proton separation compared with the pseudization radius.

I do not need to calculate high energy un-occupied orbitals of H2, I was just checking my chosen
pseudopotentials and other parameters on some simple gas phase molecules before I start some
adsorption studies on surfaces.

I will attempt my calculation with the all-electron pseudopotential you linked to and see what happens.

Steven Best
PhD Student,
School of Chemistry and Physics,

Queensland University of Technology, Brisbane, Australia

________________________________
From: users <users-bounces at lists.quantum-espresso.org> on behalf of Lorenzo Paulatto <paulatz at gmail.com>
Sent: Wednesday, December 22, 2021 6:05 AM
To: Quantum ESPRESSO users Forum <users at lists.quantum-espresso.org>
Subject: Re: [QE-users] Unoccupied molecular orbitals near the vacuum level.

Dear Steven,
Any pseudopotential (and PAW is no different) will eventually fail for sufficiently high energy. In the specific case of H2, it is worst because the two protons are closer than the typical pseudization radius. I would bet that generating a hard pseudopotential with a couple of high energy projectors could solve the problem, but I have no proof.

Also the "all-electron" pseudopotential available here should work (beware, it requires a cutoff of 500+ Ry)
http://quantum-espresso.org/upf_files/H.coulomb-ae.UPF<https://urldefense.com/v3/__http://quantum-espresso.org/upf_files/H.coulomb-ae.UPF__;!!NVzLfOphnbDXSw!RNcP52_WIFjau2sC7gx0qofDjhmmgaoBPgbUzG-OS-H_bR6ZkTxdKExOgUnaqP7YBnuaEgal$>

--
Lorenzo Paulatto

On Tue, Dec 21, 2021, 09:06 Steven Best <steven.best at hdr.qut.edu.au<mailto:steven.best at hdr.qut.edu.au>> wrote:
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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