The original lead developer of MOLCAS (Bjorn Roos) also happened to be a pioneering force behind the initial development of a lot of those methods in the first place, see the answer here for more information on that: Is there a free package with robust CASSCF functionality?. It can do many of the things most mainstream quantum chemistry packages can do: integrals, Hartree-Fock, DFT, MP2, coupled cluster, geometry optimization, etc., and while it can do "single-reference quantum chemistry" about as well as the other mainstream packages (or in many cases even better, which I'll mention later in this answer!), it has also been very popular in the "multi-reference" community because of its implementations of CASSCF, RASSCF, GASSCF, CASPT2, RASPT2, etc. MOLCAS is now in it's 5th decade! The first version was written in the 1980s by the group of Bjorn Roos, one of history's giants in quantum chemistry.
PS: You can find more choices from this Wikipedia link.
Due to the user-friendly style, ORCA is considered to be a helpful tool not only for computational chemists but also for chemists, physicists, and biologists that are interested in developing the full information content of their experimental data with help of calculations. ORCA uses standard Gaussian basis functions and is fully parallelized. It can also treat environmental and relativistic effects. It features a wide variety of standard quantum chemical methods ranging from semiempirical methods to DFT to single- and multireference correlated ab initio methods. It is a flexible, efficient, and easy-to-use general-purpose tool for quantum chemistry with specific emphasis on spectroscopic properties of open-shell molecules. ORCA is a general-purpose quantum chemistry program package that features virtually all modern electronic structure methods (density functional theory, many-body perturbation, and coupled-cluster theories, and multireference and semiempirical methods). As I have not been in contact with matter modeling for many years, I would like a suggestion from the community so as not to have to break my promise, honoring my late advisor. Long story short, recently I am considering returning to computational chemistry studies and I am not aware of software that can replace Gaussian. By that time, I had just left the academic world, but in any case I promised myself that I would not use Gaussian software anymore.
However, he deeply regretted doubting the anonymous community of scientists who created. At the time he was already retiring and was not too worried (he died in 2018). A few months later, he was surprised to receive a notification (when it was time to renew the license, if I remember correctly) that both he and his coworkers were no longer allowed to use Gaussian.
He didn't care much about it, said it was possibly a hoax and openly defied Gaussian's licensing terms because he thought he would not be punished and that the scientists who created the anonymous website were just disseminators of fake news.Įventually came the day when my advisor published an article in which he compared the computational efficiency of Spartan with that of Gaussian in simulating a PAH he was studying. regarding the banning of researchers involved in the development of competing software (there is a very famous paper in Nature about that). When I started studying computational chemistry ( circa 2007), my supervisor used to tell me about the controversy surrounding Gaussian, Inc.