Computational Chemistry, orca, programs

Calculations with Orca

This is a very quick post on how to run a calculation in Orca. In this case, it is an optimization of the iridium pentachloronitrosyl anion. This was a post that I wrote for my research group to encourage the use of Orca for quantum mechanical calculations. I will write this up further in due course.

For more detailed information, check out the Orca manual and the Orca Input Library.

For example:

# comments start with a ‘#’
! opt pal4 bp86 def2-svp def2-svp/j slowconv tightscf printbasis ecp{def2-sd=[Ir], def2-tzvp, def2-tzvp/j} #command line
*xyz -1 1 #coordinates are given in xyz coordinates (and in Angstroems), followed by the charge and spin of the complex.
Ir -1.90921 0.68572 0.12837 #the coordinates are here
Cl -3.64939 1.02673 1.70334
Cl -3.41543 -0.48246 -1.28291
Cl -0.40315 1.85449 1.53933
Cl -0.16921 0.34529 -1.44692
Cl -1.33590 -1.34990 1.20205
N -2.39619 2.41486 -0.78364
O -2.66228 3.35972 -1.28198
* #end with an asterisk

The line of commands (reproduced below) states that this is a DFT calculation and that we want to optimize (opt) the complex using the GGA functional BP86.

In addition, we want to use the def2-svp basis set (which is a split-valence basis set). Slowconv and tightscf calculations are useful for transition metals and control the convergence of the SCF.

Finally, there is a ECP on Ir, indicated by def2-sd = [Ir], which places an ECP of the Stuttgart-Dresden type on the 60 core electrons of iridium. The next part, ‘def2-tzvp’, indicates that the valence electrons of Ir are treated by a triple-zeta basis. In summary, Ir has an ECP with triple-zeta valence and the other atoms are treated with the spit-valence def2-SVP basis. The commands def2-tzvp/j and def2-svp/j are ‘density fitting’ basis sets, and these allow the program to make some approximations that make the calculation run faster.

! BP86 DEF2 opt-svp slowconv tightscf printbasis ecp DEF2-sd = {[Ir], DEF2-tzvp, DEF2-tzvp/j} #command line

Use:

*xyz A B

to declare the coordinates in xyz format. A and B are the charge and spin, respectively. Below that, type in the coordinates. Finally, end the file with a star and save it with the extension ‘inp’ (but that is not strictly necessary).
To calculate, open a terminal, navigate to the file, and run the file (obviously, replacing ‘filename’ with the name of your file):

orca nameOfFile.inp > nameOfFile.out &

or you could use a script.

One more thing.
If you want to make a calculation run in parallel, put:

pal8

in the command line. In this example, it is for a computer with 8 cores – if necessary, change it.
Of course, this is not a large calculation, but the calculation runs very fast, ending on my computer in a little less than 3 minutes running on two threads on two cores of my laptop.

 

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