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I have been doing some research into computational chemistry, having never looked into the area before, and have read about a popular, paid-for software "Gaussian", and free alternatives such as "GAMESS". From what I understand they are both capable of employing methodologies such as PM3, but I am struggling to understand the point of the software itself; how is a Gaussian PM3 calculation any different to a GAMESS calculation? There must be a reason one would want to pay for it.

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  • $\begingroup$ related chemistry.stackexchange.com/questions/27599/… $\endgroup$ – Mithoron Apr 22 '16 at 16:22
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    $\begingroup$ One difference (that has little to do with your query but is relevant) is that software developed by a commercial, for-profit operation in some cases will be better maintained and more portable than similar software forged in the open-source community: you're not paying for functionality as much as you're paying for support, documentation, development by professional software engineers, and other tangibles that reduce your own costs in terms of time and money. $\endgroup$ – Todd Minehardt Apr 22 '16 at 16:29
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    $\begingroup$ GAMESS is not open source, it is free for academic use. Beyond that, there are a lot of implementation that Gaussian has that GAMESS does not, and vice versa. Researchers in the computational chemistry branch often utilise many different programs. A lot of them are not free. Gaussian also is often a good choice for beginners, since it comes with a more comprehensible user interface and documentation. Also the user community is much larger. And try to search for problems with GAMESS online - it's a nightmare because of actual games. $\endgroup$ – Martin - マーチン Apr 22 '16 at 16:53
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    $\begingroup$ Gaussian includes a lot more methods within one nicely done package and, frankly speaking, is faster. Paid developers do better job and have paid time to implement more advanced techniques in the product. However, since gaussian is proprietary, it is not always beneficial to employ it, and for many simpler tasks investment into hardware is more reasonable. When I checked last time, gaussian allowed calculation of analitical frequencies for dft methods, while gamess didn't. It meant, that for any large molecule gamess couldn't produce IR spectra in reasonable time using DFT approach. $\endgroup$ – permeakra Apr 22 '16 at 21:15
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    $\begingroup$ @permeakra if you pay enough you can read the source code of Gaussian, too. Both programs are not open source software, because that would mean that you can modify and redistribute the code to anyone. Both are proprietary software, they come with serious limitations in licencing. $\endgroup$ – Martin - マーチン Apr 23 '16 at 0:46
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There are several interrelated reasons (disclaimer: I am a GAMESS developer).

Gaussian has a lot of features and interoperability. To take your PM3 example. Yes, the results of a PM3 calculation are identical in GAMESS and Gaussian. But until recently you couldn't do a PM3 calculation in solution using the PCM method in GAMESS. Also, what if you want to do a PM6 calculation instead. In GAMESS this is currently only possible only for elements up to F.

Because of this Gaussian has been the program of choice for many people, which also means they have trained a lot of people to use Gaussian. Learning to use a new (and perhaps free) program well can be very time consuming, so it might be more cost effective to pay for a program you know how to use well. And chances are you probably have to learn how to use several new programs to get all the functionality of Gaussian.

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