# Monthly Archives: October 2015

## Comparing the Relative Stability of Non-Equivalent Molecules

How do you compare the stability of two or more compounds which differ in some central atom(s)?

If you simply calculate the energy of both compounds you get a misleading answer since the number of electrons is different from one to the next -in fact, the answer is not so much misleading as it is erroneous. Take compounds 1 and 2 shown in figure 1, for example. Compound 1 was recently synthesized characterized through X-Ray crystallography by my friend Dr. Monica Moya’s group; compound 2 doesn’t exist and we want to know why – or at least know if it is relatively unstable respect to 1. Figure 1. Compound 1 exists but compound 2 is apparently less stable. Is it?

Although stoichiometry is the same, varying only by the substitution of Ga by Al the number of electrons is quite different. We then made the following assumption: Since the atomic radii of Ga and Al are quite similar (according to the CCDC their respective covalent radii are 122 and 121 pm), relative stability must rely on the bonding properties rendering 2 harder to obtain, at least through the method used for 1. The total energy for compound 1 was calculated at the M06-2X/6-31G(d,p) level of theory; then both Al atoms were changed by Ga and the total energy was calculated again at the same level. Separately, the energy of isolated Ga and Al atoms were calculated. Compensating the number of electrons was now a simple algebraic problem:

ΔE = E(MnBxOy) – nEM + nEM’ – E(M’nBxOy)

The absolute energy difference E1 – E2 is staggering due to the excess of 36 electrons in 2. But after this compensation procedure we now have a more reliable result of ΔE value of ca. 81 kcal/mol in favor of compound 1. In strict sense, we performed geometry optimizations at various stages: first on compound 1 to remove the distorsions due to the crystal field and then on the substituted compound 2 to make sure Ga atoms would find a right fit in the molecule but since their covalent radii are similar, no significant changes in the overall geometry were observed confirming the previous assumption.

We now have the value of the energy difference between 1 and 2 and other similar cases, the next step is to find the distal causes of the relative stability which may rely on the bonding properties of the Ga-O bond respect to the Al-O bonds.

What do you think? Is there another method you can share for tackling this problem? Please share your thoughts on the comments section.

## Restarting Calculations from rwf Files – Gaussian

Having a long calculation terminated just because it ran out of time in the queue is very frustrating; even more so if restarting it from the last accesible point is hard to do.

I have recently performed some particularly demanding calculation: Basis Set Superposition Error (BSSE) with the Counterpoise method and second order Moller-Plesset perturbation theory calculation (MP2). The calculation ran out of time but I was able to restart it because I had the rwf file! My input looked a bit like this:

`#p mp2/GEN counterpoise=2 maxdisk=200GB`

So here is how it works.

The very first line of your calculation gives you the process ID number which is not necessarily the same as the PID given by the queue system (in fact, is not the same because the latter corresponds to the submitted script, not the instructions in it i.e. your calculation)

``` Entering Gaussian System, Link 0=g09
Initial command:
/opt/SC/aplicaciones/g09-C.01/l1.exe /tmpu/joaqbf_g/joaqbf/Gau-38954.inp
-scrdir=/tmpu/joaqbf_g/joaqbf/
Entering Link 1 = /opt/SC/aplicaciones/g09-C.01/l1.exe PID=     38955.
```

(emphasis in red is mine)

This is the number you want to write down. You will need to find the corresponding rwf file (usually in your SCRATCH directory) as Gau-PID.rwf (in the aforementioned case, Gau-38955.rwf). If you are a bit paranoid like myself you want to copy and keep this file safe but be aware that these are very long files, in my case it was 175 GB long. Now you need to launch your calculation again with the following input:

```%rwf=myfile.rwf
%nosave
%chk=myfile.chk

Title Card

# restart

rest of input```

You can add all other controls to the Link0 section such as %nprocshared or %mem according to your needs.

I’m pretty sure it should work for other kinds of calculations in which taking from the checkpoint file is not as easy, so if you run into this kind of problems, its worth the try.