Rosarium Philosophorum on Computational Chemistry

Every year, the Israel Journal of Chemistry publishes the special “Rosarium Philosophorum” – a unique issue that aims to curate the recollections, perspectives, and visions of thought-leaders in a particular field. Such a challenge seems ambitious at first glance: how can one possibly portray in one issue the enormity of the field of Computational Chemistry, trying to encompass both its depth and its breadth? As we wrote to the authors at the start of this process many months ago, we wanted each one of them to contribute with a distinctive piece of this giant puzzle. It is our hope that these pieces have indeed come together to illustrate part of the beauty, scope, and importance of this field we are all devoted to. Editing and compiling this issue offered us the chance to reflect on what exactly computational chemistry is. People often confuse theoretical and computational chemistry. This confusion is understandable since demarcating these two fields is hopeless. In simplistic terms, theoretical chemistry develops formulas and models, while computational chemistry computes with a computer. However, nowadays, most theoreticians perform computational simulations and most computationalists theorize models and formulas, blurring the boundaries between the two. Moreover, when done correctly, computational chemistry means doing experiments, even if they are virtual. In this sense, computational chemistry truly is experimental chemistry (just much cleaner). Meanwhile, many in vitro experimentalists are learning the basics of in silico experimentation, and many computationalists have one foot in a wet lab, again blurring these arbitrary boundaries. In other words, computational chemistry is as integral a part of the chemical world as any other chemical field, and it is integrated in an interdisciplinary way. Interestingly, combining computational chemistry with other disciplines has generated new subfields, such as computational organic chemistry, computational biochemistry, computational electrochemistry, etc. As a result, one can perceive computational chemistry either as a central discipline that branches into those fields, or branches of other fields. One way or the other, in the year 2022, computational chemistry is a prolific, indispensable, and wonderful scientific endeavor, and we are honored and proud to have edited this special Rosarium Philosophorum. Despite the difficulty in classifying the field of computational chemistry by any individual parameter, we will make a rough (and arbitrary) chronological division, outlining two particular timeframes: one of the pioneers, and one of the users. The age of the pioneers began with the first “super” computers (and by “super” we mean having the computational power of an old smartphone1), where only qualitative, niche-style research was feasible. This was a challenging and fascinating era, defined by punch cards and floppy disks, by Hartree-Fock and extended Hückel, by Fortran and Crays. It was also defined by a small number of valiant out-of-the-box-thinking researchers who had to learn not only theoretical methods and models, but also their limits, so as to make chemically insightful discoveries by walking the thin line that the low accuracy of the methods provided them. They struggled not only against the limited computational power and the lack of good approximations to Schrödinger's equation, but also against many (if not most) experimentalists who were hesitant and suspicious of this new field. Then came the age of established users. By this, we do not mean that currently there are no worthy developments; however, we must acknowledge that the 21st century provides the privilege and ease of working on proven methods with stable software packages. Accuracy is at hand (if you know what you are doing), and you can reach it with the click of the mouse. Personal computers and the internet are the norms, as are DFT, Linux, and colorful graphical user interfaces (or command-line scripting for the brave) – all of these making our lives noticeably easier. It is noteworthy that now we also put to work many creative tools that helped us (arguably) understand the greatest paradigm of chemistry, the chemical bond. But perhaps the most significant advantage of this age is that experimentalists finally accept and acknowledge our work. Many of them will even seek help and guidance from computationalists! Still, there are many hurdles on the road ahead, and as we move to larger and more accurate studies, we never lack challenges. There are plenty of undiscovered effects and phenomena waiting to be revealed. Nonetheless, the heavy bulldozing that paved this road was gracefully carried out for us by the pioneers. One might say that we have now entered the third age. In recent years, we are witnessing an increase in predictions instead of a posteriori explanations; computations have even provided corrections to wrongfully assigned experimental characterizations (learning from the old, remarkable Foster and Boys' methylene affair2). However, the greatest leap is taken toward developing the tools for the complete and/or extremely fast automatization of computational chemistry processes. We are, of course, speaking about artificial intelligence and big data, but other exciting directions are also being charted as we write these words. Some groups are creating the framework for the autonomous exploration of complete reaction surfaces and chemical spaces, and some are tinkering with quantum dynamics to mimic the natural events in the flask. Some might even heretically discuss whether the wet lab will still make sense in the near future. Meanwhile, haptic technology and quantum computing might (or might not) disrupt our workstyle and understanding of computational chemistry. In this issue we try to reflect on the past, describe the present, and predict the future. As fitting the subjective and artistic task at hand, authors were given a free hand to express themselves, which you will see in the diversity of formats, lengths, and topics laid out in the current issue. Nevertheless, by necessity, many stones remained unturned. We are proud of having curated this small collection of many viewpoints, containing historical and personal perspectives, some predictions of novel directions, some classical, and some unorthodox stances about our beloved discipline. We invite the reader to agree or disagree, ponder, and discover old and new pieces of the computational chemistry universe.