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##

Lattice dynamics, via the force constant approach

VASP supports the calculation of lattice vibrations in
the harmonic approximation. One caveat is that large supercells (with several hundred atoms)
are required, and a high precision is very desirable.
We recommend the following setup.

PREC = Accurate
LREAL = .FALSE # real space projection .FALSE. or Auto
ISMEAR = 1 # tetrahedron method with Bloechl corrections
SIGMA = 0.2
IBRION = 6

It is safer to avoid real space projection, since it introduces
small errors that can change low frequency modes by several %.
Also `PREC= A` is preferable over `PREC= Normal`, in particularly
for large unit cells. The timestep (`POTIM`) defaults to a reasonably
small value of 0.015, although tests might be required if very short
bonds are present in the system (e.g. hydrogen).
Alternatively linear response theory can be used for density functional theory
calculations:
PREC = Accurate
LREAL = .FALSE # real space projection .FALSE. or Auto
ISMEAR = 1 # tetrahedron method with Bloechl corrections
SIGMA = 0.2
IBRION = 8

Results should agree within 1 % with the finite difference code,
although small errors in the force constant will affect low frequency
modes more strongly than high frequency modes.
It is strongly recommened to relax
all atoms in the supercell carefully and accurately before the
phonon-calculation i.e. using

PREC = Accurate
LREAL = .FALSE # real space projection .FALSE. or Auto
ISMEAR = 1 # tetrahedron method with Bloechl corrections
SIGMA = 0.2
IBRION = 1
EDIFF = 1E-6

This relaxation run must be performed with identical parameters
as the phonon calculation.
Obviously you do not want to spoil your results, because
of finite forces for the initial positions.

** Next:** Pseudopotentials and PAW potentials
** Up:** Examples
** Previous:** Simulated annealing
** Contents**
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N.B. Requests for support are to be addressed to: vasp.materialphysik@univie.ac.at