# [Pw_forum] PBE0 in PWSCF

xinzheng li ucanxli at ucl.ac.uk
Thu Mar 5 19:28:52 CET 2009

```Dear friends,

My name is Xinzheng Li. I am a postdoc in University College London. I
am using PWSCF to perform some PBE0 calculations for water cluster on
metal surface recently. In order to make sure that the PBE0 number I got
from PWSCF is correct. I performed a test for the binding energy of a
water-dimer in gas phase (total energy of 2 water moleculars minus the
total energy of a water dimer) taking the all-electron PBE0 results from
Gaussian03 as reference.  The converged binding energy from  Gaussian03
is around 207 meV. While after PBE0 calculation of PWSCF is performed, I
have these lines in the output file:

!    total energy              =   -68.48225787 Ry
Harris-Foulkes estimate   =   -72.38797788 Ry
estimated scf accuracy    <     0.00000008 Ry

The total energy is the sum of the following terms:

one-electron contribution =  -144.72933450 Ry
hartree contribution      =    71.54274579 Ry
xc contribution           =   -12.87564636 Ry
ewald contribution        =    13.67425725 Ry
Fock energy 1             =    -7.81160613 Ry
Fock energy 2             =    -7.81177236 Ry
Half Fock energy 2        =    -3.90588618 Ry

1. Shall I take the '!   total energy' as the final total energy, or the
'Harris-Foulkes estimate' as the final total energy? For the comment:
'The total energy is the sum of the following terms', if I sum the
numbers below, I don't get either 'Harris-Foulkes estimeate', or '!
total energy'. Only if I sum the one-electron contribution, hartree
contibution, xc contribution and the ewald contribution, can I get the
'Harris-Foulkes estimate'. Does it mean that I should take
'Harris-Foulkes estimate' as the final energy? If so, what is the
meaning of the '! total energy'?

3. If I take Harris-Foulke estimate' as the total energy, the binding
energy I get is 184 meV. If I take '! total energy' as total energy, the
binding energy is 158 meV. Both of these two number are > 20 meV from
the Gaussian all-electron benchmark. What is the expectation for the
accuracy of such a value in the PWSCF code compared with all-electron
Gaussian (when pseudopotentials, and supercell-size convergence are
well-tested)?

Thanks a lot for your patiency in advance!

Sincere
Xinzheng

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Dr. Xinzheng Li
London Centre for Nanotechnology, University College London
17-19 Gordon Street, London WC1H, 0AH
ucanxli at ucl.ac.uk
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