Dear Colleagues,

Professor Linus Pauling (1901–1994) was an incredible scientist, able to give fundamental contributions in several branches of science that, thanks to his intuitive understanding and genius, were demonstrated to be largely interrelated. Disciplines, such as structural, inorganic and quantum chemistry, X-ray and electron crystallography, spectroscopy, biochemistry, structural biology and molecular genetics will owe him an immense gratitude forever.

He was widely regarded as the greatest chemist of the twentieth century and has been called by the New Scientist one of the 20 greatest scientists of all time. He was also rated in 2000 as the 16^th most important scientist in history. No less important has been his work as a peace activist. Based on his intuition that radiation could be harmful for the genome (even if in small amounts), he fought determinedly for interrupting nuclear tests, due to the development of huge nuclear arsenals after the Second World War. Though it is now common knowledge, he had an intuition on possible radiation effects that had, at that time, been considered an outrageous, if not even a subversive idea.

Linus Pauling is the only person to have been awarded two unshared Nobel Prizes—the Nobel Prize in Chemistry in 1954 for his scientific work, and the Nobel Peace prize, just eight years later, for his peace activism.  Besides the two Nobel Prizes, he received over 50 medals and awards from a great variety of organizations and almost as many honorary degrees from universities around the world.

The Nobel Prize in Chemistry was awarded to Pauling for “his research into the nature of the chemical bond and its application to the elucidation of the structure of complex substances”. Discovering the fascinating relationship between structure and chemical bonding has always been his main scientific target—an interest originated from his initial X-ray diffraction structural works and fuelled by the beauty and power of the emerging quantum mechanical theory and of its application to chemistry, of which he becomes soon a pioneer and one of the major and successful actors in the world. All of this is clearly reflected in his famous book on “The nature of chemical bond”. First published in 1939, this book has formed generations of chemists since then.

There are three important reasons for launching in 2020 a Commemorative Special Issue honouring Linus Pauling. Exactly one hundred years ago, late in 1920, Pauling published his first paper on the structure/chemical bond relationships, by formulating a simple set of rules governing the structure of ionic minerals. Then, eighty and sixty years ago, in 1940 and in 1960, the second and third editions of his abovementioned book were published. The third edition represented a large expansion over the first two, as it included several new developments, such as wide applications of the electro-neutrality principle, the equation relating bond distances and fractional bond orders that form the basis of the so called and recently revisited valence bond model, and a new theory of the structure of electron-deficient substances. He also introduced the resonating valence bond theory and the application of the valence bond theory to the electronic structure of metals and intermetallic compounds. In spite of all these new additions, Pauling was aware that the theory of chemical bonding was still in its infancy. In the introduction to the third and last edition of his book, he wrote that “the theory of chemical bond, as presented here, is still far from perfect. Most of the principles that have been developed are crude and only rarely can they be used in making an accurate quantitative prediction….”. However, he was also convinced that following Poincaré “it is far better to foresee even without certainty that not foresee at all”. About 60 years later, the situation has clearly improved. Yet, as written by G. Frenking and S. Shaik in their preface to a Special Issue of the Journal of Computational Chemistry, dedicated to G. N. Lewis, the founder of the modern electronic theory of valence and the most influential inspirer of Pauling, “we are still far from understanding the nature of the chemical bond and the field itself is exploding with new problems to be solved! The dialog of chemists with the notion of the chemical bond continues”.

It is with this situation of affairs in mind that it is our great pleasure to invite our colleagues working in the broad field of structure and chemical bonding to participate to this Linus Pauling commemorative issue. We envisage contributions from several disciplines having chemical structure (geometrical and electronic) and its understanding, rationalization and prevision through chemical bonding as one of the key ingredients. Scientists developing or making use of either experimental or theoretical methods and scientists analysing chemical bonding within formalisms based on Hilbert space entities or defined in the real space (quantum chemical topological methods) are highly welcome to contribute to this issue. Papers on recent developments in the valence bond theory and models are also eagerly awaited, as well as on any experimental or theoretical progress able to shed light on chemical bonding issues. Applications to challenging structural and chemical bonding cases, including those occurring under extreme conditions, such as high pressure, are also highly welcome. The diversity of contributions, coming from several fields of science and from scientists with different expertise, will be a faithful representation of the diversity as well as of the profound unity and interrelationship of Linus Pauling interests. He remains an inspiration to all of us and we are pleased to invite you to submit a publication for this Special Issue.

Dr. Carlo Gatti
Dr. David L. Cooper
Dr. Miroslav Kohout
Dr. Maxim L. Kuznetsov
Guest Editors

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• Chemical bonding
• Structure bonding relationships
• Valence bond theory and methods
• Quantum chemical topological methods
• Valence bond models
• Chemical bonding under extreme condition
• Structural chemistry
• Structural biology
• Structural mineralogy
• Structural crystallography