Tribute to

P Biman Bagchi has made pivotal contributions to the area of dynamics of chemical and biological systems in an academic career spanning more than three decades. He has been a great teacher and mentor for a large number of young theoretical physical chemists of India and a creative and insightful collaborator with leading scientists worldwide. We take great pride in guest editing this special issue of The Journal of Physical Chemistry B in honor of Prof. Bagchi who is, arguably, the finest theoretical physical chemist India has ever produced. Prof. Bagchi has spent about 30 years at the Indian Institute of Science (IISc), Bangalore, during which he has traversed a wide landscape of physical chemistry. It is impossible to discuss all of his work; it is not even possible to mention each area he has worked in. In the following, we choose only a few selective topics and give short comments on their impact as we understand them. This is how we chose to pay our tribute to this ever-young, dynamic, and completely apolitical scientist who is so dear to all of his students, postdocs, and collaborators and will remain so forever. Professor Bagchi joined IISc, Bangalore, in 1984. His major research interests during the first ten years at IISc included dynamics of polar solvation, dielectric relaxation, and dynamic solvent effects on activated and barrierless chemical reactions. In the next ten years, starting from the mid-nineties, he primarily worked on conductance and viscosity of electrolyte solutions, dynamics of biological water, solvent relaxation near micellar surfaces, vibrational energy relaxation and dephasing, and dynamics of glassy and supercooled liquids and liquid crystals. In the next ten years, up to the present, he further extended his studies of vibrational dynamics to two-dimensional infrared spectroscopy of aqueous systems and continued to work on glassy and supercooled systems and liquid crystals. During this period, he also entered into new areas of research in chemical biology such as protein−DNA interactions, enzyme catalysis, and theory of autoimmune disorders and human immune response. Solvation dynamics was a topic of huge contemporary interest in the mid-eighties. New results were coming in from the ultrafast laser spectroscopic groups of one of us (G.R.F.), Paul Barbara, and others. The solvation dynamics was reported to have times scales faster than dielectric relaxation, which posed a challenge to theorists for proper explanations. Just before joining IISc, Bagchi, in collaboration with G.R.F. and David Oxtoby, had developed a theory of dipolar solvation using a continuummodel of the solvent and a frequency dependent dielectric function. The theory predicted a solvation time that was faster than the dielectric relaxation time of the solvent, thus providing an explanation of the experimentally observed fast relaxation of the time dependent solvation energy. The continuum theory was subsequently generalized by Bagchi and co-workers in many different directions such as incorporation of multi-Debye relaxation, non-Debye relaxation, inhomogeneity of the medium around a solute, etc. Still, being based on continuum models, all these extensions lacked the molecularity of the solvent. Besides, these theories considered only the rotational motion of solvent molecules because the dynamics came through the frequency dependence of the long wavelength dielectric function. Microscopic theories based on molecular solvent models also started coming from other groups; however, these microscopic theories also included only the rotational motion of solvent molecules. These rotation-only microscopic theories predicted an average solvation time that was longer than the long-wavelength continuum theory predictions due to contributions from finite wavevector processes. Subsequently, a microscopic theory was developed by the Bagchi group that included translational contributions of solvent molecules in addition to their rotational motion. The theory showed that the polarization at finite wavevectors can relax at a faster rate in the presence of finite translational contributions. This was indeed an important result because, in many dynamical processes, it is the finite wavevector response of the solvent that matters most. Hence, the overall solvation can occur with a time scale faster than that predicted by rotation-only theories but in better agreement with then available experimental results. Subsequently, the theory was further extended to incorporate many subtle dynamical aspects such as inertial effects, viscoelastic effects, self-motion of the solute, and the connections between the so-called microscopic and macroscopic orientational relaxation times, etc., which lead to quantitative agreements of the theoretical predictions with the experimental results for a variety of solute−solvent systems. The theories of orientational and dielectric relaxation were also used in arriving at microscopic theories of dielectric friction. A self-consistent theory was developed for the dielectric friction and dielectric relaxation where it was shown that the presence of translational contributions can make dielectric relaxation more Debye-like for cases where only rotational contributions give rise to a highly non-Debye form of dielectric relaxation. The possible roles of solvent dynamics on rates of chemical reactions constituted another topic of huge interest in the eighties and also in years to follow. The celebrated Grote−Hynes