ORCA 6.1 Foreword

It was less than one year ago, July 2024, that ORCA 6.0 was released to the general public by a proud and happy but also thoroughly exhausted development team. As we pointed out at the time, ORCA 6.0 was less an update to an existing program but rather a completely re-imagined quantum chemistry program suite in which large parts of the massive code base where re-designed and rewritten from scratch. This process took nearly three years and resulted in a program that was much leaner, much cleaner and as a result also more modern and more efficient.

Our promise at the time of the release of ORCA 6.0 was that the new infrastructure would allow for much more rapid and more confident method development than ever before. With the release of ORCA 6.1, we believe that we made good on this promise. Despite less than one year has passed since the release of ORCA 6.0, version 6.1 is packed with new features and further significant improvements under the hood. This release can therefore indeed be regarded as a proof-of-principle for the new infrastructure. It turned out to be so encouraging that we have every intention to keep up the pace and aim at a release of a new ORCA version approximately every 12 months – hold us to it!

ORCA 6.1 is the result of the work of many individuals. The bulk of the work has been done by the development teams at the Max Planck institute für Kohlenforschung in Mülheim an der Ruhr, Germany and at FACCTs in Cologne, Germany. However, we also very much enjoy our increasingly numerous collaborations with other research groups around the world - thank you so much, you rock!

I want to express my gratitude to all the many people who have put their enthusiasm and hard work into making this release reality. All of them were, are and will continue to be instrumental for the success of what now has become a very large project! An ORCA release would not be possible without the ORCA development team, led by Frank Wennmohs and featuring Ute Becker, Dagmar Lenk, Dimitrios Liakos, Kantharuban Sivalingam and Axel Koslowski, who have done invaluable work in making sure that the code stays healthy, our libraries are up to date, that it compiles on all platforms (and give correct results everywhere), educate the developers on good programming style, fixing countless bugs, parallelizing everything and going after each and every deviation that our massive test suite reports.

Alexander Auer and Markus Bursch have taken the lead on the important but incredibly tedious task to restructure the mammoth that the ORCA manual has become, making sure that it is up to date and available in HTML and PDF versions. Thank you so much! The importance of that work cannot possibly be overemphasized.

It gives me enormous pleasure to observe the increasing impact on industrial computational chemistry that FACCTs has under the outstanding leadership of Christoph Riplinger. This not only leads to rapid and sustainable growth of the company but also provides many important ideas and incentives for exciting science. The development team at FACCTs continues to make many important contributions to the code, exciting new features, new methods, new workflows together with an unwavering commitment to precision and excellence. I should specifically mention Bernardo de Souza, Georgi Stoychev, Miquel Garcia-Rates, Ingolf Harden, Nicolas Foglia and Hagen Neugebauer. Thank you all – it is a pleasure!

Working on an ORCA release is not a chore, it is a labour of passion, joy and dedication. It gives me incredible joy to work on something that proves to be useful for tens of thousands of people all around the globe. It also gives me incredible joy to work on this as part of a team that is enthusiastic, highly motivated, highly skilled and highly dedicated.

Now you, the ORCA community, holds the next chapter of our journey in your hands. We are proud of ORCA 6.1 and we promise that this won’t be our last rodeo in the continuing ORCA development. We hope that you enjoy it as much as we enjoyed working on it - and - of course, we hope that it helps you solving chemical problems, large or small, whatever your scientific pathway may be.

Frank Neese

On behalf of the ORCA development team

May 26\(^{th}\), 2025