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##

Dimers

Reproducing accurate dimer distances is an important difficult benchmark for
potential. If a potential works accurately for dimers and bulk calculations,
once can be quite confident that the potential possesses
excellent transferability.
For the simulation of the dimers, one can use the point and
displace the second atom along the diagonal direction.
Generally bonding length and vibrational frequency have to be compared
with accurate reference data.
It is recommended to perform these calculations
using the constant velocity molecular dynamic mode (i.e. IBRION=0,
SMASS=-2). This mode speeds up the calculation because
the wave functions are extrapolated and predicted using
information from previous steps.
The INCAR file must contain additional lines to perform the
constant velocity MD:

ionic relaxation
NSW = 10 number of steps for IOM
SMASS = -2 constant velocity MD
POTIM = 1 time-step for ionic-motion

In addition to the positions the POSCAR file must also
contain velocities:
dimer
1
10.00000 .00000 .00000
.00000 10.00000 .00000
.00000 .00000 10.00000
2
cart
0 0 0
1.47802 1.47802 1.47802
cart
0 0 0
-.02309 -.02309 -.02309

For this POSCAR file the starting distance is 2.56 Å, in each step the
distance is reduced by 0.04 Å, leading to a final distance of
2.20 Å. The obtained energies can be fitted to a Morse potential.
*Mind:*
In some rare cases like C, the calculation of the
dimer turns out to be problematic. For C the LUMO (lowest
unoccupied molecular orbital) and the HOMO (highest occupied
molecular orbital) cross at a certain distance, and
are actually degenerated, if the total energy is used
as variational quantity (i.e.
). Within the finite
temperature LDA these difficulties are avoided, but interpreting the
results is not easy because of the finite entropy
(for C see Ref. [54]).

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** Previous:** Determining the groundstate energ
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N.B. Requests for support are to be addressed to: vasp.materialphysik@univie.ac.at