[Pw_forum] binding energy for water adsorption at certain surface

[vega lew]

Dear all,

I want to calculate the binding energy for water adsorption at certain
surface. So I first put an water molecule at some sites of the surface, and
relaxed the whole systems( water and the upper half of the surface slab,
lower slab was fixed). After  relax calculations finished, I copy the slab
atoms and the water molecule coordinates separately into two inputfiles(the
dimension of the supercell, and other parameters such as k point sampling,
ecut, et al. were not changed) and performed a SCF calculation to get the
single point energy. Then I substrated the two single point energy (water
molecule and the surface slab) from the final energy for the whole system
reported by former relaxed calculation.
this procedure was quite well when the adsorptions were in a molecular
style. But if the water molecule dissociated at the surface, the subsequent
SCF calculations of the dissociated water molecules were not converged. How
could I cope with this? To increase the ecut, k-point, empty bands? to
decrease the broadening, mixing beta? Or setting the mixing mode to
'local-TF'? If the convergence could be achieved by doing this, do you think
I could use the this value to calculate the  binding energy directly while
the scf calculation of the slab and the former relax calculation were using
different parameters (such as ecut, number of k-points,  mixing beta et
al.)?

the input file of the scf calculation of water is show as follows (the water
was in a dissociated configuration. and surface slab was  removed to perform
scf calculation),

 &CONTROL

                       title = '*********_water'
,
                 calculation = 'scf'
,
                restart_mode = 'from_scratch'
,
                      outdir = '/tmp/' ,
                      wfcdir = '/tmp/'
,
                  pseudo_dir = '/home/vega/espresso-4.0/pseudo/' ,
                      prefix = '********_water'
,
                     disk_io = 'none'
,
                       nstep = 1000
,
 /

 &SYSTEM

                       ibrav =
8,
                   celldm(1) =24.8624,
                   celldm(2) = 0.8520,
                   celldm(3) = 1.6964,
                         nat =
3,
                        ntyp =
2,
                       nosym = .ture.
,
                   ecutwfc   =   30,
                   ecutrho   =  300,
 /

 &ELECTRONS


 /


ATOMIC_SPECIES
# Ti   47.86700  Ti.pw91-sp-van_ak.UPF
    O   15.99940   O.pw91-van_ak.UPF
    H            1.00794
H.pw91-van_ak.UPF
ATOMIC_POSITIONS crystal
O       -0.672041076   0.333142157   0.526904777
H       -0.486792757   0.330778459   0.505140407
H       -0.739833562   0.333472512   0.544633120
K_POINTS gamma

thank you for reading.

any hints on my questiosn will be deeply appreciated.

vega

-- 
==================================================================================
Vega Lew ( weijia liu)
PH.D Candidate in Chemical Engineering
State Key Laboratory of Materials-oriented Chemical Engineering
College of Chemistry and Chemical Engineering
Nanjing University of Technology, 210009, Nanjing, Jiangsu, China
******************************************************************************************************************
Email: vegalew at gmail.com
Office: Room A705, Technical Innovation Building, Xinmofan Road 5#, Nanjing,
Jiangsu, China
******************************************************************************************************************
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