OpenMolcas is a quantum chemistry software package developed by scientists and intended to be used by scientists. It includes programs to apply many different electronic structure methods to chemical systems, but its key feature is the multiconfigurational approach, with methods like CASSCF and CASPT2.

OpenMolcas is not a fork or reimplementation of Molcas, it is a large part of the Molcas codebase that has been released as free and open-source software (FOSS) under the Lesser General Public License (LGPL). Some parts of Molcas remain under a different license by decision of their authors (or impossibility to reach them), and are therefore not included in OpenMolcas.

Latest references:

• "OpenMolcas: From Source Code to Insight." J. Chem. Theory Comput. 15 (2019) 5925-5964. doi:10.1021/acs.jctc.9b00532

• "Modern quantum chemistry with [Open]Molcas." J. Chem. Phys. 152 (2020) 214117. doi:10.1063/5.0004835

For more detailed information, please refer to the wiki pages.

OpenMolcas is configured with CMake. A quick way to get it up and running is the following:

1. Clone the repository:

   git clone https://gitlab.com/Molcas/OpenMolcas.git
   

2. Get the lapack submodule (only needed if you don't use another linear algebra library like MKL or OpenBLAS):

   cd OpenMolcas
   git submodule update --init External/lapack
   cd ..
   

3. Create a new directory and run cmake from it:

   mkdir build
   cd build
   cmake ../OpenMolcas
   

4. Compile with make:

   make
   

5. Run the verification suite (failures in "grayzone" tests are expected):

   pymolcas verify
   

For running other calculations you should define the MOLCAS environment variable to point to the build directory. Run pymolcas --help to see the available options of the script. In particular it is recommended to run:

pymolcas -setup

for your first installation.

The documentation can be found in the doc directory, you can read it in HTML format or PDF format. Note that most of it precedes the creation of OpenMolcas and it is probably outdated in several points. It may also mention features not available in OpenMolcas.

OpenMolcas is a community-supported software and as such it doesn't have an official technical support. If you have any problems or questions, you can use the Issues page or the Molcas forum, and hopefully some other user or developer will be able to help you.

If you need technical support, you can acquire a Molcas license.

Since OpenMolcas is FOSS, you can download it, modify it and distribute it freely (according to the terms of the LGPL). If you would like your contributions to be included in the main repository, please contact one of the developers, write a message in the forum or submit a merge request. Everyone is welcome to send patches, suggestions and bug reports, but please let us know if you would like to be a "developer" member of the Molcas group.