# Theory and Computation for Constructing Molecules Characteristic of Heavier Elements and Nanostructures

Dr. Shigeru Nagase has made a wide range of original contributions in theoretical and computational chemistry. He has predicted, designed, and developed a variety of interesting molecules prior to or in cooperation with experiment. It should be emphasized that his theoretical investigations lead to a number of very important or challenging experimental targets. He has played an important role in promoting chemical researches by close interplay between theoretical prediction and experiment. His main achievements are summarized in the following way.

** 1. Bonds, Structures, and Reactions Provides by Heavier Elements**

Dr. Nagase has made remarkable contributions to developing and enriching heavier main group chemistry. He has presented fundamental rules to explain why heavier main elements provide bonds and structures that differ from those formed by second-row elements represented by carbon. Based on these rules, he has investigated a number of novel polycyclic, polyhedral, multiply bonded, aromatic, and hypervalent molecules, which contain heavier main group elements in the frameworks, and predicted how these molecules are synthetically accessible and isolable in stable forms. These theoretical predictions have been widely accepted as a very important guide to experiment. It is notable that almost all of the predicted molecules have eventually been synthesized and isolated by world-leading experimentalists. He has also investigated interesting interactions between heavier main group elements and transition metals. Recently, he has developed novel reactions characteristic of heavier main group elements, which invalid the well-known Woodward-Hoffmann rule. In collaboration with experiment, he has found that small molecules such as H_{2} and NH_{3} are activated under mild conditions by multiple bonds and divalent species of heavier Group 14 elements, without transition metal catalyses. These enrich the chemistry of heavier elements.

**2. Nanomolecular Systems**

Dr. Nagase has performed many pioneering studies on nanomolecular systems. He is the first to determine the structures and electronic properties of endohedral metallofullerenes. He pointed out that the majority of long-believed structures of endohedral metallofullerenes are completely wrong. This impacted the chemistry of metallofullerenes as a surprising landmark. He is also the first to indicate that the isolated pentagon rule established as an iron law in fullerene chemistry are invalidated upon endohedral metal doping, thereby a new research field being opened. He suggested and verified that the positions and rotational motion of encapsulated metals lead to unique electronic and magnetic properties, which are controllable by exohedral addition and can be used as a molecular device. In addition, many theoretical investigations have been carried out for the electronic properties and reactivities of endofullerenes, carbon nanotubes, carbon peapods, and nanocables as well as cage-like silicon and germanium clusters stabilized by transition metals. These are of great importance in nanoscience.

**3. Quantum Chemistry Calculations with High Speed and Accuracy**

Although density functional theory (DFT) is widely used to calculate large molecules as well as small molecules, the generally used DFT methods fail to describe noncovalent interactions that play an essential role in nanomolecular systems. In contrast, second-order Møller-Plesset perturbation theory (MP2) is the simplest and effective to take account of noncovalent interactions at an ab initio level. However, MP2 calculations becomes much more time-consuming than DFT calculations, as the size of molecules increases. Therefore, MP2 calculations have been highly speeded up by developing new parallel algorithms and the applications to very large molecules have been made practically possible. In addition, efficient periodic boundary MP2 calculations have been developed with new theory, which are applicable to large periodic systems. These developed programs show much higher computational efficiency than any other programs and enrich the applications of MP2 calculations. In addition, high parallel fragment molecular orbital and divide-and-conquer calculations have been realized to make MP2 calculations feasible for huge molecules. For very accurate calculations, a new projector Monte Carlo method has been developed by sampling configuration state functions as walkers, which makes possible the accurate calculations of excited state as well as ground states. The accuracy is systematically improved by increasing the number of walkers and exact energies are obtained for a given basis set.

Dr. Nagase has made outstanding contributions in the development of heavier element and nanomolecular chemistry as well as theoretical and computational chemistry. Therefore, it was recognized that his achievements are worthy of the Chemical Society of Japan (CSJ) Award.