openqp

What OpenQP computes

Open Quantum Platform, abbreviated OQP and pronounced oh-coop, is a source available quantum chemistry program. The README centers it on mixed reference spin flip time dependent density functional theory, written as MRSF-TDDFT. The program combines Hartree Fock and DFT references with correlated wavefunction methods, linear response excited states, nuclear derivatives, reaction paths, nonadiabatic dynamics, and QM/MM calculations inside a single package. That combination means a computational chemist can study both ground state structure and excited state behavior without switching tools. The project is maintained by the Choi Group at KNU, and the README links a journal article that introduces OpenQP in the Journal of Chemical Theory and Computation in 2024. The README also points to a separate tutorials site and a manual site for guided calculations and for method, keyword, and build references. Because OpenQP is source available rather than fully open under a single standard license in the current metadata, a user should read the repository license file and the licensing guide before redistributing. The breadth of included methods places OpenQP in the category of research grade electronic structure software, where the value is in handling difficult cases like states that simpler models describe poorly. A reader should expect to compile and run the program on a workstation or cluster, using the manual for the keywords that select each calculation type. Windows, macOS, and Linux are all supported according to the README.

The MRSF-TDDFT approach

The README explains why MRSF-TDDFT is the heart of OpenQP. It keeps the linear response structure of standard time dependent DFT while removing the spin contamination that limits conventional spin flip TDDFT. Spin contamination is a known problem where unrestricted calculations mix in wrong spin states, and OpenQP's mixed reference design avoids that. The result, according to the README, is a consistent description of multiconfigurational ground and excited states, which covers diradicals, bond breaking, conical intersections, photochemical processes, and spin orbit coupling. These are exactly the hard cases in photochemistry and materials where a single reference method fails. By retaining linear response, MRSF-TDDFT stays close enough to familiar TDDFT that existing intuition carries over, while the mixed reference extends the range. The README lists MRSF-TDDFT with a ROHF mixed reference using DTCAM series functionals, and it credits specific papers for the functionals and the method. The available calculations include ground and excited states, analytic gradients, properties, non adiabatic coupling and its gradient form, spin orbit coupling, optimization, and dynamics. For a researcher modeling light driven reactions or bond cleavage, this method list shows OpenQP targets the parts of quantum chemistry that are most sensitive to spin and correlation errors. The method is also the basis for the runtype equal namd nonadiabatic dynamics path that the README describes as specific to MRSF-TDDFT.

Methods and documentation

Beyond the main MRSF-TDDFT method, the README functionality table lists related approaches. UMRSF-TDDFT is an unrestricted mixed reference based on UHF orbitals, used for excitation energies in unrestricted mixed reference calculations. MRSF-EKT applies the extended Koopmans' theorem to MRSF states to give ionization and electron attachment energies, Dyson orbitals, and pole strengths. SF-TDDFT uses a spin flip TDA from a high spin reference, while TDHF and TDDFT provide RPA and TDA response, and Hartree Fock, density functional theory, MP2, coupled cluster, configuration interaction, CASSCF, and multireference PT2 cover the rest of the wavefunction toolbox. The documentation strategy is split into two public sites. The OpenQP tutorials site offers guided calculations, which help a new user reproduce a known result step by step. The OpenQP manual site holds method descriptions, keyword references, and build instructions, which are the references a user consults when setting up a real job. The README itself only sketches the program, so the manual is where build details live. With Fortran as the primary language and a list of contributors that includes the Choi Group and several international collaborators, OpenQP has an active development base. A user planning serious work should start at the tutorials to learn the input style, then move to the manual for the keywords that control each electronic structure method listed in the table. Installation is available through pip install openqp or a source checkout.

Editorial conclusion

OpenQP is written in Fortran and combines HF and DFT references with correlated wavefunction methods, excited states, and QM/MM in one program. Its documentation lives in a tutorials site and a manual site linked from the README, and version 1.3.0 and later use a dual licensing model with a free research license and a separate commercial license.