[Pw_forum] problem in calculating the binding energies with pwscf

[elbuesta at icqmail.com]

Hello Joerg Buchwald, 

a few comments. Starting magnetization should be between -1 and 1. 1 means that all available electrons from the PP have spin up. Also, if you want to do spin unrestricted calculations, you should set nspin=2. Only starting magnetization does not do anything. 

In addition, I wouldn't compare the energy of the bulk and the isolated atom this way because the cutoff on the WFC is quite different for each case: energy differences converge much faster than the total energy. But I don't think that is the problem here, only the magnetization is.

I slightly modified your inputs and got 4.5eV. If I remember well, this value is okay. I hope it helps. All best!

Free Si atom:

&control
    prefix='si_free',
    pseudo_dir='/home/alex/pseudo'
    outdir = '/scratch/alex',
 /
 &system
    ibrav=  1, celldm(1) =25, nat=  1, ntyp= 1,
    ecutwfc = 30.0,occupations='fixed',nspin=2,tot_magnetization=2.,nosym=.true.,
 /
 &electrons
 /
ATOMIC_SPECIES
 Si  28.086  Si.pbe-n-van.UPF
ATOMIC_POSITIONS
 Si 0.50 0.50 0.50
K_POINTS gamma
......................................

Bulk Si:

&control
    prefix='silicon',
    pseudo_dir='/home/alex/pseudo'
    outdir = '/scratch/alex',
 /
 &system
    ibrav=  2, celldm(1) =10.263142, nat=  2, ntyp= 1,
    ecutwfc = 40.0,
 /
 &electrons
 /
ATOMIC_SPECIES
 Si  28.086  Si.pbe-n-van.UPF
ATOMIC_POSITIONS
 Si 0.00 0.00 0.00
 Si 0.25 0.25 0.25
K_POINTS automatic
 9 9 9 1 1 1

................................................................


-----Original Message-----

From: Jörg Buchwald <joerg.buchwald at iom-leipzig.de>
To: pw_forum <pw_forum at pwscf.org>
Sent: Thu, Jul 21, 2011 3:15 pm
Subject: [Pw_forum] problem in calculating the binding energies with pwscf


Hello,
I'm performing some first test-calculations with pwscf, where I try to
get the binding energy of bulk silicon. The problem is, that the
resulting energy, which should be the difference of the total
energy of silicon crystal structure and the a free atom, is about 3eV
too low. I used different pseudopotentials, different
lattice-parameters and I also tested the convergencies (with k and
the energy-cut-off) which worked, but I don't have a clue what a made
wrong.

Here are my Input files:

---
 &control
    prefix='silicon',
    pseudo_dir='/home/joerg/espresso-4.3.1/pseudo'
    outdir = '/home/joerg/scratch',
 /
 &system
    ibrav=  2, celldm(1) =10.263142, nat=  2, ntyp= 1,
    ecutwfc = 30.0,
 /
 &electrons
 /
ATOMIC_SPECIES
 Si  28.086  Si.pbe-n-van.UPF
ATOMIC_POSITIONS
 Si 0.00 0.00 0.00
 Si 0.25 0.25 0.25
K_POINTS automatic
 9 9 9 1 1 1
---

and for the free atom:

---
&control
    prefix='silicon',
    pseudo_dir='/home/joerg/espresso-4.3.1/pseudo'
    outdir = '/home/joerg/scratch',
 /
 &system
    ibrav=  0, celldm(1) =50.2, nat=  1, ntyp= 1,
    ecutwfc = 80.0,occupations='fixed',starting_magnetization(1)=10,
 /
 &electrons
 /
ATOMIC_SPECIES
 Si  28.086  Si.pbe-n-van.UPF
ATOMIC_POSITIONS
 Si 0.50 0.50 0.50
K_POINTS gamma

CELL_PARAMETERS {cubic}
    1.00 0.00 0.00
    0.00 1.00 0.00
    0.00 0.00 1.00
---

It would be nice if someone could help me.
Thx & regards,
Jörg Buchwald


Leibniz-Institut fuer Oberflaechenmodifizierung e.V.
Permoserstrasse 15
04318 Leipzig
GERMANY

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