[QE-users] Ice vibrational density of states

[p ul]

Dear QE users,
I am trying to reproduce the vibrational density of states of ice. As a
first attempt I simulated a single water molecule in a large cubic cell
with pw.x . Then I caluculated the vibrational modes with ph.x and then,
after q2r.x, I used matdyn.w. The vibrational density seems consistent as
there are delta functions in correspondence of the bending and
antisimmetric stretching frequencies (as it is a single molecule and not
crystal). The main problem is that in the vibrational density of states
there is not a trace of the symmetric stretching that is present even in
the normal mode calculation.
The structure has been also relaxed and different energy precision has been
used for dos calculation.
here the input file used and a plot where the symmetric stretching at ca.
3800 cm-1 is missing and the bending and asymmetric stretching present.
-----
pw.x
------
prefix = 'h2o'
pseudo_dir = '../pseudo'
outdir= './out'
/
&SYSTEM
ecutwfc = 120
ibrav = 1
celldm(1) = 20
nat = 3
ntyp = 2
/
&ELECTRONS
conv_thr = 1.D-9
/
&IONS
/
ATOMIC_SPECIES
H 1 H_ONCV_PBE-1.0.upf
O 16 O_ONCV_PBE-1.0.upf
ATOMIC_POSITIONS angstrom
O 4.7571 4.0000 3.902317
H 5.5142 4.0000 4.488417
H 4.0000 4.0000 4.488417

K_POINTS (automatic)
1 1 1 0 0 0
-------
ph.x
-------
phonon calc
&inputph
outdir = './out'
prefix = 'h2o'
ldisp = .true.
amass(1) = 1
amass(2) = 16
nq1= 1, nq1 =1, nq3=1
fildyn = 'h2o.dyn'
/
---
q2r.x
------
&input
fildyn = 'h2o.dyn'
zasr = 'simple'
flfrc = 'h20.fc'
/
-------
matdyn.x
-----------
&input
asr = 'simple'
dos = .true.
amass(1) = 1
amass(2) = 16
flfrc = 'h2o.fc'
fldos = 'h2o.phdos'
nk1= 2, nk2 = 2, nk3 = 2
/
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