H2O vibration

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Task

Calculation of the vibrational frequencies of a $\mathrm{H}_{2}\mathrm{O}$ 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

• It is strongly recommended to set the energy cutoff manually in the INCAR file,

as it gives you more control over the calculations and forces you to think about this aspect.

• For the ionic optimisation the DIIS algorithm is used.

This algorithm builds an approximation of the Hessian matrix and usually converges faster than the conjugate gradient algorithm. It is, however, recommended to specify the number of independent degrees-of-freedom manually.

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

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