When I started studying computational chemistry (circa 2007), my supervisor used to tell me about the controversy surrounding Gaussian, Inc. regarding the banning of researchers involved in the development of competing software (there is a very famous paper in Nature about that). 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 bannedbygaussian.org were just disseminators of fake news.

Eventually 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. 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. At the time he was already retiring and was not too worried (he died in 2018). However, he deeply regretted doubting the anonymous community of scientists who created bannedbygaussian.org. 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.

Long story short, recently I am considering returning to computational chemistry studies and I am not aware of software that can replace Gaussian. 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.

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    $\begingroup$ I wonder what features of Gaussian people like the most and then we can think about finding a software that will do better than Gaussian. $\endgroup$ – QMlab Mar 20 at 22:39
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    $\begingroup$ Comments on this question have been moved to the GAUSSIAN chat room. @QMlab you're welcome to share your ideas there too! $\endgroup$ – Nike Dattani Mar 21 at 22:19


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. 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. (in addition to MRCI). 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?. Additionally, it can do QM/MM (molecular dynamics with quantum effects included) and supports solvent models like COSMO, so it's not only useful for small molecules but also proteins and other macromolecules.


  • Free, open source, and open development (here's the Git page) since 2017!
  • While free, it has the "quality" of an expensive commercial package like GAUSSIAN, since it was commercial for decades.
  • Big user community, big development community worldwide, decades of history but with lots of work done to keep it modern and up-to-date.
  • Has multiple interfaces to use advanced and modern state-of-the-art methods which have been developed recently and are still being developed, for example you can do:
    • FCIQMC (via the interface to NECI)
    • DMRG (via the interfaces to several DMRG codes: Block, QCMaquis, CheMPS2)
    • Uncontracted MRCI, MR-ACPF, MR-AQCC (via the interface to COLUMBUS)
    • Molecular Dynamics with Surface Hopping (via the interface to SHARC)
  • There's GUI tools such as LUSCUS, MolGUI, GV
  • Even for basic things like integrals, and geometry optimization, which every competing code needs: MOLCAS and OpenMOLCAS have the best-quality software. For example the commercial software MOLPRO (known as one of the fastest and most well-developed programs for electronic structure) uses the SEWARD routine by Roland Lindh (current lead developer of MOLCAS) which was originally written for MOLCAS, because it might be the fastest code for integrals written, and allows basis sets with angular momentum far higher than most other packages. Likewise, for geometry optimization, MOLCAS has advanced features which were again borrowed in the MOLPRO package.


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). 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. 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 can also treat environmental and relativistic effects.

ORCA uses standard Gaussian basis functions and is fully parallelized. 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.

  • Which can ORCA do?

    • geometry optimizations;
    • spectroscopic parameters;
    • Hartree Fock, DFT...

PS: You can find more choices from this Wikipedia link.

Hope it helps.

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    $\begingroup$ +1 for ORCA. Being (in its current form) on of the younger codes also has huge advantages for the output it creates. It is very clear and legible. Also, whenever this pandemic is over, see if you can attend an ORCA user group meeting and meet the core people of the ORCA team. $\endgroup$ – DetlevCM Mar 20 at 9:16


Psi4 is a fully open-source quantum chemistry package that is gaining a lot of interest in my community (computational) combustion chemistry. Here is the manual for the latest release: it has most, if not all, of the theoretical methods (HF, DFT, CC,...) and also lots of interfaces and integrates into C++ and Python (sadly no Fortran).

The bonus on it being open-source is that it eliminates licensing headaches while also making it far easier to interact with the developers (or make our own contributions) and integrate it into other toolchains, e.g. running Psi4 automatically (integration in-progress into ARC) in the context of automatically generating mechanisms (overview).



For many researchers, GAMESS is the "natural" substitute for GAUSSIAN. Developed and maintained by the group of prof. Gordon (also a banned GAUSSIAN user, see image below).

GAMESS is a program for ab initio molecular quantum chemistry. Briefly, GAMESS can compute SCF wavefunctions ranging from RHF, ROHF, UHF, GVB, and MCSCF. Correlation corrections to these SCF wavefunctions include Configuration Interaction, second order perturbation Theory, and Coupled-Cluster approaches, as well as the Density Functional Theory approximation. Excited states can be computed by CI, EOM, or TD-DFT procedures. Nuclear gradients are available, for automatic geometry optimization, transition state searches, or reaction path following.

  • What can GAMESS does? (full list of capabilities here)

    • Calculates RHF, UHF, ROHF, GVB, or MCSCF self-consistent field molecular wavefunctions.
    • Calculates the electron correlation energy correction for these SCF wavefunctions using a) Density Functional Theory (DFT), b) Valence Bond Theory (VB) c) Configuration Interaction (CI), c) Many Body Perturbation Theory (MP2), e) coupled-cluster (CC) or Equation of Motion CC (EOM-CC) methodologies. f) for MCSCF, generates R12 basis set corrections by an interface to the MPQC package.
    • Calculates semi-empirical MNDO, AM1, or PM3 models using RHF, UHF, ROHF, or GVB wavefunctions.
    • Computes the energy hessian, and thus normal modes, vibrational frequencies, and IR intensities. Raman activities are a follow-up option.
    • Computes excited state energies, wavefunctions, and transition dipole moments at various levels.
  • License agreement: https://www.msg.chem.iastate.edu/gamess/License_Agreement.html

It has versions for Linux (ARM and x86_64 processors), MacOS X, and Windows among others.



NWChem aims to provide its users with computational chemistry tools that are scalable both in their ability to treat large scientific computational chemistry problems efficiently, and in their use of available parallel computing resources from high-performance parallel supercomputers to conventional workstation clusters.

NWChem software can handle:

  • Biomolecules, nanostructures, and solid-state
  • From quantum to classical, and all combinations
  • Ground and excited-states
  • Gaussian basis functions or plane-waves
  • Scaling from one to thousands of processors
  • Properties and relativistic effects
  • Brief list of capabilities: (full list here)
    • Both Gaussian type and plain wave basis sets.
    • Calculations on periodic systems
    • Energies and analytic gradients for RHF, UHF, ROHF, DFT, R-MP2, U-MP2, CASSCF
    • Analytic second derivatives for RHF, UHF and R-DFT
    • Excited state calculations (CIS, TDHF, TDDFT)
    • Energies with iterative CCSD, CCSDT, CCSDTQ (also EOM-CC) for RHF, UHF and ROHF
    • RI-MP2 for RHF and UHF references
    • Vibrational SCF and DFT for anharmonic corrections
    • Conductor Like Screening Model (COSMO) solvation
    • ONIOM hybrid calculations
    • Molecular dynamics(MD) and hybrid QM/MM calculations
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    $\begingroup$ I wonder why people do not like NWChem, for me it is the best code for DFT related calculations, energy, td-dft, molecular magnetic properties, transition state search, geometry optimization. It has relativistic methods only scalar but 2-component approaches as well. Truly massively parallel capabilities. Flexibility to choose different functionals. $\endgroup$ – QMlab Mar 21 at 20:20
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    $\begingroup$ @QMlab, From what I have heard, NWChem's algorithms are specifically targeted for massively parallel calculations, so it doesn't perform as well as other programs for most of the small calculations we do (<8 cores). Maybe that's one of the reasons NWChem is not as popular. $\endgroup$ – Shoubhik R Maiti Mar 21 at 21:49

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