Dear Felix,
First of all, the "hydrogen becomes metallic" probably means that you
should apply spin polarisation as you have only one atom. Please remember
that the absolute energies do not make sense, the total energy of the atom
should be calculated by removing the energy difference between a pseudo
atom and the all-electron atom, but this is prone to numerical
(convergence) errors.
Second, dissociating a water molecule by pulling one hydrogen apart
would in principle lead to a double radical; well, in practise it would
lead to two non-neutral constituents (in one calculation the electrons can
move freely, in "metallic language" the Fermi level is global and same
everywhere, even if the atoms would be 1 km apart). And further, most of
the approximations, in particular GGAs, have a problem when pulling apart
atoms, they give wrong asymptotics (famous examples are dissociating H_2).
As I do not know what your target goal is, I would once more look at the
literature on DFT and these asymptotic failures, whether the DFT (and the
XC functional that you plan to use) is able to capture the correct
asymptotics.
Greetings from the Austrian Alps,
apsi
PS Indeed, the quantity you would be calculating is rather dissociating
energy, or?
-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-=*=-
Ari Paavo Seitsonen / [email protected] / http://www.iki.fi/~apsi/
Ecole Normale Supérieure (ENS), Département de Chimie, Paris
Mobile (F) : +33 789 37 24 25 (CH) : +41 79 71 90 935
On Fri, 26 Feb 2016, Felix Brockherde wrote:
Dear all,
We would like to calculate the binding energy for non-equilibrium H2O
geometries with PAW pseudo potentials.
The initial thought was to calculate the single-atom energies. However, for H
the system becomes metallic. We used smearing but the results seem too far from
the experimental
value for us to use them (-0.91784682 Ry).
Now, we would like to calculate the binding energy by separating the H2O
molecule as far as necessary. We would thus avoid inaccuracies caused by
smearing because it is not a
metallic system. However, when pulling H2O apart, the DFT calculations do not
converge.
Can anybody help with either how to get the separated H2O converged or a
different approach to get the binding energy with PAW pseudo potentials?
We used PWSCF v.5.1.1 and I attached an input file where QE does not converge.
Best regards,
Felix Brockherde
TU Berlin
&CONTROL
calculation = 'scf',
wfcdir = 'tmp',
pseudo_dir = '/home/user/pseudo_potentials',
tprnfor = .true.,
prefix = 'prefix',
tstress = .false.,
outdir = './',
wf_collect = .true.,
/
&SYSTEM
nat = 3,
ntyp = 2,
ecutwfc = 90,
ecutrho = 360,
celldm(1) = 30,
ibrav = 1,
/
&ELECTRONS
/
ATOMIC_SPECIES
H 1.00794 H_pbe-kjpaw_psl.0.1.UPF
O 15.9994 O_pbe-kjpaw_psl.0.1.UPF
ATOMIC_POSITIONS alat
O 0.5 0.3 0.5
H 0.5 0.7 0.3
H 0.5 0.7 0.7
K_POINTS automatic
1 1 1 0 0 0
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