We know that [NiCl4]2- can potentially take on two different geometries: tetrahedral and/or square planar.
Following the procedure established for the tetrahedral [NiCl4]2- determine the energy of the square planar [NiCl4]2- (this will have D4h symmetry)
You will need to:
compute square planar [NiCl4]2- at the 3-21G level, on the local computer, don't forget that the square planar structure is a singlet!
this optimisation can be a bit problematic because of the D4h symmetry so add into the additional keywords section "opt=z-matrix". Look at the key words page if you want a bit more information about this option. When you come to save the job make sure the "save as cartesian coordinates" check-box is NOT selected, do this for the higher level calculation as well.
use the 3-21G optimised geometry to start a 6-31G(d) structure, resymmetrise the molecule first, and don't forget the extra keywords (you will have three extra keywords now, "opt=z-matrix int=ultrafine scf=(conver=9)". Carry out this calculation on the SCAN server.
carry out a frequency analysis to confirm you have a minima and a population analysis for the MOs and NBO charges. When you set up the frequency job make sure you remove the "opt=z-matrix" extra keyword (as you are not doing an optimisation!)
Normally structures are optimised in cartesian coordinates, however in some special cases optimising the structures in terms of bond distances and angles is a better method and the "opt=z-matrix" switches this second option on
With the options outlined above these jobs should take less than 10min each, once your job is running if yours takes more than 10min (in the Wall Time column), do NOT wait for it to finish as you will block the queue for others, DELETE the job from the queue and go back and check that you have implemented the above options correctly. If you still have problems ask a demonstrator, don't repeat jobs without understanding what the problem is.
Your 6-31G(d) job should converge fine, however when you do the frequency analysis you should find that you get one negative frequency!
This is a good example of how a converged structure can "appear" to be correct but is actually a transition state. Your negative frequency should be about -31 cm-1.
A small negative frequency like this is an indicator that your basis set is not quite good enough and that you need to improve it, or it could be a real transition state ... you won't know until you do the higher level calculation.
Animate the negative frequency ... can you see how two Cl go up and two Cl go down, this mode is turning the square planar structure into a tetrahedral structure!
As this is only a small frequency you can use the output of your 6-31G(d) optimisation to start the 6-311G(d,p) job. Use use the 6-31G(d) optimised geometry to start a 6-311G(d,p) structure with the extra keywords "opt=z-matrix int=ultrafine scf=(conver=9)" activated. Carry out this calculation on the SCAN server.
Carry out a frequency analysis on your optimised 6-311G(d,p) structure to confirm you have a minimum and to obtain a population analysis for the MOs and NBO charges. Make sure you remove the "opt=z-matrix" extra keyword (as you are not doing an optimisation!)
When the 6-31G(d) and 6-311G(d,p) jobs have completed successfully record the unique DOI for submission as part of the lab.
You are now ready to move onto the next part of the lab
