[Pw_forum] PBE0 in PWSCF

[xinzheng li]

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

I have two questions about this output:

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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