H2O vibration: Difference between revisions
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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, | 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 {{TAG|ENCUT}} to | 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 == | ||
Revision as of 12:51, 6 June 2012
- 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 1 ! one k-point rec ! in units of the reciprocal lattice vector 0 0 0 1 ! 3 coordinates and weight
- 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
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.
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