[Pw_forum] Lattice constants with vdW-DF/vdW-DF2

[Ari P Seitsonen]

Dear Colleagues,

   I want to calculate molecules adsorbing on metallic surfaces, and since 
the adsorption energy of them is know to be small, I would like to apply 
the vdW-DF/vdW-DF2 for the calculations. When I calculate the lattice 
constant with those functionals, I seem to get very large lattice 
constants, 5-10 % larger than the experimental one. An example for bulk, 
fcc gold is attached. I perform the calculation looping over different 
values for the lattice constant; the result seems to be independent of the 
cut-off energies (in this case I have large values for 'ecutwfc'/'ecutrho' 
because this was the last run for the convergence tests).

   Thus my question, does any one see something wrong in my input? Since 
the exchage part of the exchange and correlation is treated with a GGA 
functional (independently whether I use vdW-DF2 and PW86 or vdW-DF and 
revPBE) and correlation with LDA, plus an attractive(?) long-range 
contribution, I would expect lattice constants much closer to the 
experiments.

     Greetings from Wroclaw,

        apsi

PS Apologies if the topic of my mail is an FAQ/discussed already!

-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-
   Ari P Seitsonen / Ari.P.Seitsonen at iki.fi / http://www.iki.fi/~apsi/
   Physikalisch-Chemisches Institut der Universitaet Zuerich
   Tel: +41 44 63 54 497  /  Mobile: +41 79 71 90 935
-------------- next part --------------
 &control
    calculation     = 'scf'
    restart_mode    = 'from_scratch'
    prefix          = 'bulk-Au'
    pseudo_dir      = '/users/aps/usr/espresso/PP_LIBRARY/'
    outdir          = './scratch/'
    tstress         = .true.
    tprnfor         = .true.
    verbosity       = 'high'
    etot_conv_thr   = 1e-6
    forc_conv_thr   = 1e-5
 /

 &system
    ibrav           = 2
    a               = 4.60

    nat             = 1
    ntyp            = 1
    nbnd            =  9
    occupations     = 'smearing'
    smearing        = 'fermi-dirac'
    degauss         = 0.0036749326

    ecutwfc         = 80
    ecutrho         = 800

    input_dft       = 'vdW-DF2'
 /

 &electrons
    conv_thr        = 1e-10
    mixing_beta     = 0.7
 /

ATOMIC_SPECIES
Au  1.00  Au.bp-van_ak.UPF

ATOMIC_POSITIONS crystal
Au  0 0 0

K_POINTS AUTOMATIC
 3*32  0 0 0