H2O vibration: Difference between revisions

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*INCAR
{{Template:At_and_mol - Tutorial}}
SYSTEM = H2O vibration
PREC = A
# IBRION = 1 ; NSW = 10 ; NFREE = 2 ; EDIFFG = -1E-4
ENMAX = 400
ISMEAR = 0    # Gaussian smearing
IBRION = 6    # finite differences with symmetry
NFREE = 2    # central differences (default)
POTIM = 0.015 # default as well
EDIFF = 1E-8
NSW = 1      # ionic steps > 0


*KPOINTS
== Task ==
 
Calculation of the vibrational frequencies of a <math>\mathrm{H}_{2}\mathrm{O}</math> molecule.
 
== Input ==
 
=== {{TAG|POSCAR}} ===
<pre>
H2O _2                                                                         
1.0000000
  8.0000000  0.0000000  0.0000000
  0.0000000  8.0000000  0.0000000
  0.0000000  0.0000000  8.0000000
  1    2
cart
  0.0000000  0.0000000  0.0000000
  0.5960812  -0.7677068  0.0000000
  0.5960812  0.7677068  0.0000000
</pre>
 
=== {{TAG|INCAR}} ===
{{TAGBL|SYSTEM}} = H2O vibration
{{TAGBL|PREC}} = A
# {{TAGBL|IBRION}} = 1 ; {{TAGBL|NSW}} = 10 ; {{TAGBL|NFREE}} = 2 ; {{TAGBL|EDIFFG}} = -1E-4
{{TAGBL|ENMAX}} = 400
{{TAGBL|ISMEAR}} = 0    # Gaussian smearing
{{TAGBL|IBRION}} = 6    # finite differences with symmetry
{{TAGBL|NFREE}} = 2    # central differences (default)
{{TAGBL|POTIM}} = 0.015 # default as well
{{TAGBL|EDIFF}} = 1E-8
{{TAGBL|NSW}} = 1      # ionic steps > 0
 
=== {{TAG|KPOINTS}} ===
  Gamma-point only
  Gamma-point only
   1        ! one k-point
   0
  rec      ! in units of the reciprocal lattice vector
  Monkhorst Pack
   0 0 0 1  ! 3 coordinates and weight
  1 1 1
   0 0 0


*POSCAR
== Calculation ==
H2O _2                                                                         
1.0000000
  8.0000000  0.0000000  0.0000000
  0.0000000  8.0000000  0.0000000
  0.0000000  0.0000000  8.0000000
    1    2
cart
  0.0000000  0.0000000  0.0000000
  0.5960812  -0.7677068  0.0000000
  0.5960812  0.7677068  0.0000000


How many zero frequency modes should be observed and why?  
How many zero frequency modes should be observed and why?  
Line 33: Line 46:
reference results.  
reference results.  
For finite differences, are the results sensitive to the step width {{TAG|POTIM}}.  
For finite differences, are the results sensitive to the step width {{TAG|POTIM}}.  
In this specific case, errors are too large at the default
In this specific case, the drift in the forces is too large
cutoff (drift in forces), and it is simplest to increase the
to obtain the zero frequency modes "exactly", and it is simplest to increase the
cutoff {{TAG|ENCUT}} to 600 eV. The important and physically meaningful
cutoff {{TAG|ENCUT}} to 800 eV. The important and physically meaningful
frequencies are however insensitive to the choice of the cutoff.
frequencies are, however, insensitive to the choice of the cutoff.


== Download ==
== Download ==
[http://www.vasp.at/vasp-workshop/examples/1_8_H2Ovib.tgz 1_8_H2Ovib.tgz]
[[Media:H2Ovib.tgz| H2Ovib.tgz]]
 
----
{{Template:At_and_mol}}
[[VASP_example_calculations|To the list of examples]] or to the [[The_VASP_Manual|main page]]


[[Category:Examples]]
[[Category:Examples]]

Latest revision as of 13:46, 14 November 2019

Task

Calculation of the vibrational frequencies of a molecule.

Input

POSCAR

H2O _2                                                                          
1.0000000
  8.0000000   0.0000000   0.0000000
  0.0000000   8.0000000   0.0000000
  0.0000000   0.0000000   8.0000000
   1    2
cart
  0.0000000   0.0000000   0.0000000
  0.5960812  -0.7677068   0.0000000
  0.5960812   0.7677068   0.0000000

INCAR

SYSTEM = H2O vibration
PREC = A
# IBRION = 1 ; NSW = 10 ; NFREE = 2 ; EDIFFG = -1E-4
ENMAX = 400
ISMEAR = 0    # Gaussian smearing
IBRION = 6    # finite differences with symmetry
NFREE = 2     # central differences (default)
POTIM = 0.015 # default as well
EDIFF = 1E-8 
NSW = 1       # ionic steps > 0

KPOINTS

Gamma-point only
 0
Monkhorst Pack
 1 1 1
 0 0 0

Calculation

How many zero frequency modes should be observed and why? Try to use the linear response code (IBRION=8 and EDIFF=1E-8) to obtain reference results. For finite differences, are the results sensitive to the step width POTIM. In this specific case, the drift in the forces is too large to obtain the zero frequency modes "exactly", and it is simplest to increase the cutoff ENCUT to 800 eV. The important and physically meaningful frequencies are, however, insensitive to the choice of the cutoff.

Download

H2Ovib.tgz