RESEARCH

Frontier Theoretical Development on Molecular Electric Properties

• Highly accurate theories have been developed by us taking into account the complex, correlated motion of electrons in molecules for the description of nonlinear electric properties. These 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. Extensive development of these theories was done for molecules, which have closed shell configurations. The codes have been developed after extensive testing. The codes have potential use in the description of nonlinear molecular materials, with possible applications in electronic devices.
• At the next stage, the more demanding cases of open shell systems, which are marked by high degree of quasi-degeneracy were addressed. This creates physical problems, which are theoretically difficult to address. Using a multi-determinant description of reference space, which can address this quasi-degeneracy adequately, coupled-cluster analytic derivative was formulated to compute accurate nonlinear properties. This general-purpose analytic derivative formulation is the first one based on multi-reference coupled-cluster method and is a significant development in quantum chemistry. We have implemented the theory to study properties of radicals and excited states.
• The scope of work in the area of nanoparticles synthesis in organic medium consists controlling the size distribution, understanding their assembly etc. Similarly preparation of water dispersible nanoparticles is imperative for their applicability in biological applications. Here we have been focusing on bifunctional ligands that can be used as capping agents and also as capping and reducing agents for the preparation of nanoparticles.

Theoretical investigation of Hard-Soft Acid-Base relation

• Qualitative principle related to hardness and softness has recently attracted a lot of attention in chemistry, in particular, due to the role of these in the explanation of stability of chemical species and reactivity. Our group has made early contribution by an extensive ab initio verification of the principle of maximum hardness. In particular, we have identified that for asymmetric distortions of molecules, the hardness is locally maximum. Various properties of hardness and softness were studied in relation to molecular properties, like polarizability. Using these relations, we could identify some novel relations between dipole moments and polarizability. Also among the recent contributions are use of local concepts of hardness and softness to chemical reactivity. Seminal contributions have been made by us in developing new local descriptors for intra- and inter-molecular reactivities. Recently, using local hard-soft-acid-base principle, interaction energies have been calculated with the help of only local descriptors of the interacting systems. We have recently identified “Bond Deformation Kernel” (BDK) correlating with interaction-induced shifts in O-H frequencies in halide-water clusters. Central to our 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 number of atoms. Using the NFF and charge transferred to water from halide ion, a BDK has been defined, which appropriately describes the shift in OH frequency.

Study of electron-molecule scattering

• We have also made an important 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 scattering of electrons by molecules.
• Recently we have used complex-scaling method within he coupled-cluster method to describe the electron-atom resonance. A complex absorbing potential based and an approximation to this based on multi-reference coupled-cluster method to calculate resonance of molecular anions has also been developed. The procedure is based on the analytical continuation method. The advantage of analytical continuation of the Hamiltonian in the complex plane giving the direct access to the resonances parameters is that they can be represented by using L2 wave function. The essential idea underlying the complex absorbing potentials to calculate the resonances is to introduce an absorbing boundary condition in the exterior region of the molecular scattered target which results in a non-Hermitian Hamiltonian, one of the square-integrable eigenfunctions of which corresponds to the resonant state. The associated complex eigen-value 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. The approximation developed in this year involves use of complex correlated independent particle potential, which simplifies the computation scheme. In the CIP –FSMRCC method, the analytical continuation over an already correlated effective Fock space Hamiltonian has been applied. We have tested this procedure to shape resonance in C2H4, CO and Mg.

Development and Application of Molecular Dynamics

• We developed ab initio molecular dynamics using Gaussian basis sets and Born- Oppenheimer approximation to study reactions of finite sized molecules. The Gaussian basis sets are quite useful for finite sized molecules. In particular, we are studying reactions inside finite clusters of zeolites and structures of metal clusters. Our study on structure and electron localisation function of mixed metal clusters has led to the novel evidence of anti-aromaticiticity in metal clusters.
• Sn-beta zeolite has attracted recent interest due to better catalytic behaviour 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 presence of H2O2. At NCL, the structure, bonding and acidity of Sn-beta zeolite has been studied using periodic DFT and it has been demonstrated that incorporation of Sn in BEA framework reduces the cohesive energy and is n endothermic process. It has been also shown that among the T-sites, T2 site is the most probable site for Sn- incorporation. T2 site is also higher Lewis acid site in comparison to other T-sites. Theoretical analysis done at NCL also shows that Sn atom polarizes the orbitals of oxygen atoms.

Density functional response approach for molecular properties

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

Magnetic properties

• Recently, we are interested in calculation of magnetic properties of molecules using extended coupled-cluster method, which has been used successfully by us for electric property calculations. Specifically, this is used for evaluation of diamagnetic and paramagnetic susceptibility of closed shell systems. We are also working on use of multi-reference based coupled-cluster theories for open shell systems.

Application to problems of chemical physics

• We have used our expertise as well as standard quantum chemistry techniques to important problems in chemical physics. One of the application areas has been the area of catalysis. Using various techniques, the modeling of catalytic properties of zeolites was addressed by energy calculation as well as use of concepts of hardness and softness. Weak inter-molecular interactions between small organic and inorganic molecules was also addressed. We are engaged in the application to the following areas:
  1. Structure and spectra of medium sized organic molecules by ab initio method
  2. Molecular modeling of structure and reactivity of zeolites
  3. Semi-empirical method to determine structure and reaction of organic and organo-metallic systems