A brief summary of the research activity:

Professor Debashis Mukherjee is a world-leader of Theoretical Chemistry, well-known for his pioneering researches in the field of molecular many body theory, theoretical spectroscopy and finite temperature non-perturbative many body theories. Professor Mukherjee has been the first to develop and implement a class of many-body methods for electronic structure which are now standard works in the field. These methods, collectively called multireference coupled cluster formalisms, are versatile and powerful methods for predicting with quantitative accuracy the energetics and cross-sections of a vast range of molecular excitations and ionization. A long-standing problem of guaranteeing proper scaling of energy for many electron wave-functions of arbitrary complexity has also been first resolved by him. The attractive aspects of the formalisms are compactness and high accuracy. These are now accepted as pioneering and standard works in the field. which has attracted wide international attention. He has also developed a rigorous finite-temperature non-perturbative field theory to study thermodynamics of strongly interacting many body systems, which is now being applied extensively to study dynamics of vibronic coupling at finite temperature.

Professor Mukherjee has presented his works in Plenary Lectures, Keynote Addresses and Invited Talks in all the prestigious International Conferences, including American Chemical Society Meetings, the on-going series on Recent Progress in Many-Body Theories and International Congress of Quantum Chemistry. He is frequently invited to contribute comprehensive review articles summarizing the activity of his group.

Professor Debashis Mukherjee has worked mainly in three major fields of Theoretical Chemistry:

(a) Molecular Electronic Structure and Theoretical Spectroscopy
(b) Quantum Many - Body Dynamics
(c) Statistical Field Theory of Many - Body Systems

(a) Molecular Electronic Structure and Theoretical Spectroscopy :
Professor Mukherjee has been the first to develop and implement a class of many-body methods for electronic structure which are now standard works in the field. These methods, collectively called multireference coupled cluster (MRCC) formalisms, are versatile and powerful methods for predicting with quantitative accuracy the energetics and cross-sections of a vast range of molecular excitations and ionization. The attractive aspects of the formalisms are size-extensivity, compactness and high accuracy. It has been applied to interpret e-2e spectroscopy, Auger spectroscopy and double charge transfer spectroscopy for strongly correlated molecules, where satellite structure spells a breakdown of the orbital description of the molecules These are now accepted as pioneering and standard works in the field. He also developed a linear response theory based on coupled cluster formalism (CCLRT), which is similar in scope to the SAC-CI and done independently of it. It pioneered the use of a dressed hamiltonian for energy differences, which has since been used by others. A long-standing problem of guaranteeing size-extensive theories starting with arbitrary reference functions has also been first resolved by him which has attracted wide international attention. He has also formulated an electron correlation theory for strongly correlated systems by starting from a combination of reference functions using a generalization of the usual Ursell-Meyer cluster expansion. In order to achieve this, he developed a Wick-like reduction formula using the concept of normal ordering for arbitrary reference functions. An important spin-off from the generalized Wick's theorem had been the methods of directly determining the various reduced density matrices via generalized Brillouin's theorem and the contracted Schrodinger equations. Professor Mukherjee in collaboration with Kutzelnigg developed such methods starting from his generalized Wick's theorem.

Recently Professor Mukherjee has developed state-specific many-body formalisms like coupled cluster and perturbative theories which bypass the difficulty of the notorious intruder problem for computing potential energy surfaces. These methods do not share the shortcomings of the previously used formalisms. The current applications of the methods clearly indicate the potentiality of the developments. This is considered a fundamental contribution to the molecular many-body methods, and it has attracted wide international recognition.

Professor Mukherjee has developed one of the most versatile many-body methods which can predict with quantitative accuracy the energetics, hyperfine interactions and transition probabilities of heavy atoms and ions where relativistic effects are important. These are regarded as the state-of-the art contributions in this field. He has also formulated a highly correlated coupled cluster method for understanding optical activity in atoms generated by the Parity Violating Weak - interaction, which is the one of the first theoretical formulations of this phenomena. Many comprehensive papers on these topics have elicited much interest.

(b) Quantum Many - Body dynamics :
Professor Mukherjee developed a general time - dependent perturbative theory which remains valid for arbitrarily large time range and is free from secular divergences Later, he generalized this in the many - body regime and formulated the first general time-dependent coupled cluster for wave functions of arbitrary complexity. First applications to photo-excitations and energy transfer were highly successful and have generated international attention. The method should prove to be useful to study photo-fragmentation and dissociation processes.

(c) Statistical Field Theory :
Professor Mukherjee has developed a rigorous finite - temperature field theory to study Statistical Mechanics of Many-Body systems. Unlike the traditional Thermofield Dynamics formulations, which maps a finite temperature theory to a zero-temperature one, the method has the advantage of working directly with the physical variables in the finite temperature range and is thus both more natural and compact. Applications on partition functions for strongly coupled correlated systems have shown promise of the method. A useful spin-off of the method is the combined use of time-dependent coupled cluster method and boson-mapping of stochastic variables to provide a rigorous and systematic cluster expansion method for monitoring quantum dynamics of systems strongly perturbed by colored noise. Both these formulations are now being applied extensively to study dynamics of many-body systems at finite temperature and for interpreting stochastic spectral shifts due to environmental perturbations such as coupling to bath modes or solvation shifts.