Dimers

Especially for critical cases, dimers are excellent test systems. If a pseudopotential has passed dimer and bulk calculations, you can be quite confident that the pseudopotential possesses excellent transferability. For the simulation of the dimer, one can use the $\Gamma$ point and displace the second atom along the diagonal direction. Generally bonding length and vibrational frequency have to be calculated. It is highly 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 of previous steps. Your INCAR file must contain some 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

To avoid complications use POTIM=1 for all constant velocity MD's. 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. An external self explained program called

> morse

exists.

Mind: In some rare cases like C$_2$, the calculation of the dimer turns out to be problematic. For C$_2$ 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. $sigma \to 0$). Within the finite temperature LDA these difficulties are avoided, but interpreting the results is not easy because of the finite entropy (for C$_2$ see Ref. [54]).