I know, and you know, and everybody knows Gaussian. At least you do if you still reading this. Gaussian, in its most recent version, g09, is arguably the most popular program for computational chemistry ever. Why is that so? Because it is very old, its first public version is from 1970, and fairly easy to use. However, it has a downside¹: it is not free. So, if you’re lucky you have access to a license bought by your University or other entity. But, if not, what else can you do? Well, there’s free solution, ORCA.²
The ORCA program is an ab initio, DFT, and semi-empirical SCF-MO package. It has all the most used tools for the computational chemist out there; semiempirical, HF, DFT, MP2, CCSD, you name it. It is nice, intuitive, fast, and pretty easy to use, all keyword oriented. Besides that, it has many other nice capabilities. For instance, it is specially well suited for computing molecular properties such as NMR or infrared spectres, as it page of citations indicates. Its creator, Frank Neese, is currently a director of the Max Planck Institute for Chemical Energy Conversion, in Mülheim, Germany.
You can find a lot of information about how to install and run ORCA in its manual, and for a quick reference you can check the corresponding input library site. On top of that, I would like to leave here a nice a simple input example for the program.
I like water, it was my thesis subject back in Mexico. Maybe we can try compute some stuff about the water molecule. Here we request an optimization as well as the calculation of the vibrational frequencies, at the MP2 level with the aug-cc-pVTZ basis set.
! MP2 aug-cc-pVTZ TightSCF ! Opt freq %pal nprocs 6 #number of processors used end %maxcore 4000 * xyz 0 1 O -6.816855 2.869580 0.126184 H -6.776253 3.765260 -0.477922 H -5.322674 0.992494 0.057974 *
After the exclamation mark (!) we use the corresponding keywords for the run. The %pal keyword is used in parallel runs to indicate the number of processors to use, meanwhile the %maxcore keyword indicates how much memory each processor is allowed to allocate. Finally between asterisks the coordinates of the molecule are specified. In the first line we indicate that we are using the xyz format, that the charge of the molecule is zero, and that the multiplicity is one.
And here is the result:
Pretty neat, isn’t it? I will let you assign yourself the vibrational modes that correspond to each signal in the spectra.
Did I say it was free? I lied, sorry. You don’t have to pay money, but you have to offer your soul. Just kidding. It is free, so it would be nice if you cite them if you use their work and even nicer if you let other people know about the program. Maybe they will use it and cite the work as well. All in all, a nice pyramid scheme.
1. Really, two downsides. Besides the price, the company that is behind the program now, GAUSSIAN INC. has some not so nice behavior with people that develops other electronic structure programs, i.e. it forbids them to use g09. There is such a thing as a group of computational chemist banned by Gaussian.
2. The source is not available, but the binaries are. So, not really Free Software, but you cannot have it all, really. As far as I now it is written in C+.