ORCA User Meeting

Unser Programmpaket ORCA ist ein flexibles, effizientes und leicht bedienbares universelles Werkzeug der Quantenchemie, mit besonderem Schwerpunkt auf spektroskopischen Eigenschaften von offenschaligen Molekülen. Mehr Informationen zum Programmpaket gibt es hier.

Das ORCA Programm hat mittlerweile eine weltweite, enorm wachsende Usergemeinschaft. Um Diskussionen zwischen Entwicklern, Mitarbeitern und Nutzern zu ermöglichen und zu vereinfachen, organisiert das MPI CEC im Jahr 2015 das erste ORCA User Meeting.

3rd ORCA User Meeting - 2017

The 3rd ORCA User Meeting took place from September 27-29, 2017 at the MPI CEC and featured clinics covering special topics like how to converge the SCF or geometry optimizations up to how to simulate experimental spectra and obtain accurate results.

Topics and Content

From the 27th to 29th of September, the third ORCA User Meeting took place at the MPI CEC im Mülheim an der Ruhr. This was the third time the meeting was organized be the developers of the ORCA program package to bring together the developers and the users, talk about details of the code, news in the software and how to apply electronic structure theory to its full potential.

About 20 participants had the opportunity to participate in presentations of new ORCA features, tutorials on specialized topics and discussions on how to solve typical problems in quantum chemistry. The scope ranges from absolute beginner with comprehensive "how to get started" exercisese up to advanced users that could address their detailed questions directly to the authors of the code.

There was also the possibility for lively discussion of science during the postersession. Here, an impressive range of topics from biochemistry to catalysis to materials science to spectroscopy showed how versatile applications of the code can be.

The ORCA developers team is looking forward to welcome ORCA users again for the 4th meeting in 2018 !

2nd ORCA User Meeting - 2016

The 2nd ORCA Users Meeting was aimed at providing deeper insight into all functionalities of ORCA to users. We offer special topical clinics ranging from tips for beginners, answers to frequent questions up to discussions on advanced problems. The 2nd ORCA User Meeting took place on September 5th and 6th, 2016 at the Max Planck Institute for Chemical Energy Conversion in Muelheim an der Ruhr, Germany.

More information on the ORCA User Meeting Website.

Topics and Content

On the 5th and 6th of September 2016, the MPI CEC in Mülheim an der Ruhr hosted the 2nd ORCA User Meeting.

With more than 10.000 users worldwide, ORCA has become one of the most common electronic structure codes for the calculation of molecular structures, molecular properties and chemical reactions. After the 1st User Meeting in 2015, this was the second meeting in which the developers of the program package offer the users to learn about the package, to get tips and tricks from experienced users, to discuss their problems, to express criticism and to make requests for new features.

The two-day meeting included lectures on established features, like approximate methods to speed up the calculations or overcome commonly encountered problems. In addition, recent, partially unpublished, developments where presented that open new opportunities for computational chemistry. These features will be available in the soon-to-be-released version ORCA 4.0 that is eagerly awaited by both, the users community and the developers. Topical "clinics" were also included, in which the users could get help on specific or advanced features, like how to run complicated multireference (CASSCF/NEVPT2/MRCI) calculations with ORCA, how to find transition states for chemical reactions, or how to converge difficult SCF cases.

During the poster session there were lively discussions and exchange of ideas. This also helped the developers to once more to get an impression of how ORCA is applied in the context of complex scientific problems in all fields of chemistry and physics and how to make the package even better and more versatile.

The meeting was framed by introductory remarks and concluding words by the originator of ORCA, Prof. Frank Neese, who greeted 25 enthusiastic participants. Due to the great atmosphere and the very positive feedback expressed by the users the 3rd ORCA Users Meeting will be held in 2017.

1st ORCA User Meeting - 2015

Das erste ORCA User Meeting fand am 7. und 8. September 2015 im Wissenschaftspark Gelsenkirchen statt.

On the 7th and 8th of September 2015, the first ORCA User Meeting took place in the Wissenschaftspark Gelsenkirchen. The meeting was organized by the Group for Theoretical Chemistry and Spectroscopy of Frank Neese at the MPI CEC who develop the ORCA program package. The meeting brought together authors, contributors and users of ORCA to talk about new features, current algorithms and selected applications.

Topics and Content

The meeting started with a warm welcome and an introductory talk about the past, present and future of the ORCA program package by F. Neese. In his lecture, he gave an overview of how ORCA has developed with the clear vision of applications in chemistry and spectroscopy since the early 1990s. The first Keynote lecture was held by S. Grimme from the Mulliken Center at the University of Bonn on simplified DFT methods for consistent structures and energies for large systems. S. Grimme introduced HF and DFT "-3c" approximations, which are traditional Hartree-Fock and DFT methods with minimal basis sets that include corrections for dispersion, BSSE and basis set incompleteness. Several benchmarks prove that these are computationally comparably cheap schemes that can be applied routinely and yield accurate molecular structures for very large systems.  

In the next lecture, C. Riplinger presented recent results from studies on the DLPNO-CCSD(T) approximation. Continuous improvements on the scheme and extensive benchmarks for large systems were discussed and the results demonstrate that for large systems (>500 atoms) DLPNO-CCSD(T) even has a crossover point with conventional hybrid DFT methods. Increasing the basis set size by one cardinal number increases the computational effort by a factor of ten, while it is roughly a factor of 10 to 40 more expensive to treat a 3d system compared to a linear molecule. Thus, DLPNO-CCSD(T) calculations with hundreds of atoms can be done routinely while the largest calculations carried out today feature over 10.000 basis functions.

The afternoon program started with a keynote lecture from M. Schindler of Bayer Crop Science, who talked about applications of quantum chemistry in crop protection research. He introduced the concept of quantitative structure-activity relationships (QSAR) and gave a few examples of how challenging it is for quantum chemistry to yield results which improve screening for novel compounds in crop protection. This lecture was followed by a contribution on the computational design of new battery electrolytes from M. Korth (Univ. Ulm). He outlined the challenges in battery design and how computational high-throughput screening approaches can help to find novel compounds that are well suited as electrolytes. The afternoon was concluded by a lecture from A. Hansen (Univ. Bonn) on the "Highly efficient calculation of broad range electron spectra of sizeable molecules by simplified TDA-DFT and TD-DFT methods". He discussed a novel approximation to TDA-DFT ("sTDA") based on a multipole approximation of the two electron integrals. The approximation allows for very efficient calculations of excitation spectra. These can for example be used to simulate spectra of unknown compounds and can be applied to systems as large as 800 atoms or more. The evening was spend in lively discussion at the poster session, where participants from almost ten different countries discussed their experiences with ORCA and presented their research spanning a broad range from method development to state-of-the art applications.


The second day started with a keynote lecture by F. Neese on a FCI approximation that allows for accurate calculations with large active spaces. After a discussion of the FCI problem and the CIPSI approach of Malrieu et al. he gave an outline of the configuration driven ORCA algorithm and the novel ICE-CI approximation which allow to approximate FCI accurately and efficiently for systems up to 40 electrons in 40 orbitals and will provide a powerful tool in large active space CAS-type calculations. M. Römelt (Univ. Bochum) presented his work on the computation of g-tensors for near-degenerate systems. He discussed the EPR spectroscopy of a molybdoneum model compound for nitrogen activation. He showed that the computation of g-tensors can not only help to interpret and validate experimental data, but that the information obtained also yields insight into bonding motifs and fundamental interactions in complex transition metal compounds. The morning session was concluded with a keynote lecture of R. Björnsson from the Univ. of Iceland, Reykjavik. Björnsson presented a broad spectrum of applications that can be carried out using the Chemshell environment in combination with ORCA. This allows for powerful QM/MM and dynamics applications of ORCA in conjunction with other functionalities. The long list of impressive results included QM/MM modeling of transition metals complexes in enzymes, MD studies on molecular fragmentation processes and the geometry optimization for molecular crystals.

The afternoon session started with a keynote lecture by D. Manganas (MPI CEC) on exploring topics in catalysis from a theoretical spectroscopy perspective. D. Manganas presented a cluster embedding approach which can be applied for energy and property calculations of surface species. He presented extensive DLPNO-CCSD(T) calculations for adsorption of molecular species on TiO2 and demonstrated that X-ray and Raman spectra of vanadium species can be simulated, for example using the novel ROCIS-DFT approach. After a talk focused on applications in spectroscopy, R. Izsak (MPI CEC) gave a presentation on method development, including details on the RIJ-COSX approximation and how it can be employed in novel implementations of EOM and STEOM coupled cluster for the accurate calculation of excitation energies.

The Users Meeting was concluded by F. Neese. The next ORCA Users Meeting will be held in the fall of 2016 in Gelsenkirchen and will include discussion rounds on new ORCA features, a clinic-like workshop for frequent problems aimed at novel as well as advanced ORCA users and a scientific program on novel developments and applications.

We are looking forward to seeing you next year and hope that you will give us feedback and suggestions for the next meetings!


There are now ORCA Tutorials on YouTube.

These are especially great for people starting to use ORCA.

ORCA Software

For downloading ORCA you have to register in the ORCA forum

ORCA reference manual 

ORCA 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. 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.