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# H2O vibration

## Contents

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

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.