Frontier theoretical development on molecular electric properties

These theories have been developed by taking into account the complex, correlated motion of electrons in molecules for the description of non-linear electric properties. Theories using many-body coupled-cluster methods are based on the evaluation of derivatives of energy with respect to external fields in an analytic manner. He carried out extensive development of these theories for molecules with closed shell configurations. The codes developed by him have potential use in the description of non-linear molecular materials, with possible applications in electronic devices.

At the next stage, the more demanding cases of open-shell systems, which are marked by a high degree of quasi-degeneracy, were addressed by him^[4] . This creates physical problems that are theoretically difficult to address. Using a multi-determinant description of reference space, which can adequately address this quasi-degeneracy, a coupled-cluster analytic derivative was formulated to compute accurate non-linear properties. This general-purpose analytic derivative formulation is the first one based on the multi-reference coupled-cluster method and is a significant development in quantum chemistry. He has implemented the theory to study the properties of radicals and excited states.

Theoretical investigation of hard-soft acid-base relation

Early contributions of Sourav involved an extensive ab initio verification of the principle of maximum hardness. He has studied various properties of hardness and softness in relation to molecular properties, like polarizability. Seminal contributions were made by him in developing new local descriptors for intra- and intermolecular reactivities. Using the local hard-soft-acid-base principle, he has calculated interaction energies with the help of only local descriptors of the interacting systems. He has recently identified "Bond Deformation Kernel" (BDK) correlating with interaction-induced shifts in O–H frequencies in halide-water clusters. Central to his model is the use of local polarization, which can be described by Normalized-Atom-Condensed Fukui Functions (NFF), which is the normal condensed Fukui Function multiplied by the number of atoms. Using the NFF and charge transferred to water from a halide ion, a BDK has been defined, which appropriately describes the shift in OH frequency. ^[5]

Study of electron-molecule scattering

Sourav has made a study in identifying the exchange effects as dominant contributions to the correlated static exchange (CSE) potential of the molecule in electron-molecule scattering. The properties of CSE were studied extensively in relation to their use in the scattering of electrons by molecules. Recently, his group used the complex-scaling method within the coupled-cluster method to describe the electron-atom resonance. A complex absorbing potential and an approximation to this based on the multi-reference coupled-cluster method to calculate the resonance of molecular anions have also been developed by his group. The procedure is based on the analytical continuation method. The advantage of the analytical continuation of the Hamiltonian in the complex plane, which gives direct access to the resonance parameters, is that they can be represented by using the L2 wave function. The essential idea underlying the complex absorbing potentials used to calculate the resonances is to introduce an absorbing boundary condition in the exterior region of the molecularly scattered target that results in a non-Hermitian Hamiltonian, one of the square-integrable eigenfunctions of which corresponds to the resonant state. The associated complex eigenvalue then gives the position and width of the resonance, or the auto-ionizing state. The important relaxation and correlation effects are included in the coupled-cluster method.

Density functional response approach for molecular properties

A computationally viable alternative to the full analytic response to the Kohn-Sham density functional theoretic (DFT) approach, which solves coupled-perturbed Kohn-Sham (CPKS) procedure in a non-iteratively has been formulated by Sourav. In the above procedure, the derivative of the KS matrix is obtained using the finite field, and then the density matrix derivative is obtained by a single-step CPKS solution followed by the analytic evaluation of properties. He has implemented this in deMON2K software and used it for the calculation of electric properties.^[6]

Development and application of molecular dynamics

He developed ab initio molecular dynamics using Gaussian basis sets and Born- Oppenheimer approximation to study reactions of finite-sized molecules. His study on the structure and electron localization function of mixed metal clusters has led to novel evidence of anti-aromaticity in metal clusters. Sn-beta zeolite has attracted recent interest due to its better catalytic behavior compared to Ti-Beta zeolite. Al-free Sn-beta zeolite has been recently synthesized, and it has been shown by another group to have efficient catalytic activity in Beyer-Villeger oxidation reactions in the presence of H₂O₂. The structure, bonding, and acidity of Sn-beta zeolite have been studied using periodic DFT and it has been demonstrated that the incorporation of Sn in BEA framework reduces the cohesive energy and is an endothermic process. A computational study of hydrogen storage materials, like magnesium hydrides using Born Oppenheimer molecular dynamics has been made. In particular, studies of hydrogen desorption and the effects of dopants, Al and Si have been made.^[7]