## Research

I have been involved in atomic and electronic structure calculations, developing and employing various state-of-art packages based on the formulations such as Density Functional Theory, Hartree Fock Theory, Configuration Interaction method and Solvation techniques throughout my research career.

Below are the details of various ongoing as well as past projects. Click on the titles to know more.

### New theory for Magnetism in Random Alloys

_{1-x}Mn

_{x}Pt system exhibits five magnetic structures, including a ferromagnetic phase, two antiferromagnetic phases, and two noncollinear ones [1]. The latter are characterized by superpositions of orthogonal magnetic amplitudes at two commensurate magnetic wave vectors. Here we study the magnetic interaction in this system and construct the magnetic phase diagram based on total energy calculations within the vector-DLM (disordered local moment) method representing partially ordered magnetic states in the coherent potential approximation (CPA). This method has been implemented within the linear muffin-tin orbital basis. The calculated total energies are fitted to polynomial functions of the relevant order parameters (magnetization amplitudes at the relevant wave vectors for the two components), which are subsequently used to evaluate the free energy in the mean-field approximation. The interaction between spatially orthogonal amplitudes is evaluated by noncollinear total energy calculations for representative alloy configurations. The phase diagram is constructed by minimizing the free energy at each concentration and temperature. The structure of the resulting phase diagram agrees with neutron diffraction measurements, demonstrating the applicability of the method to the studies of magnetic phase diagrams of intermetallic alloys with competing ferromagnetic and antiferromagnetic interactions.

[1] A. Z. Menshikov et al., J. Magn. Magn. Mater. 65, 159 (1987).

Published in : Phys. Rev. Lett. 115, 057203 (2015)

### Graphene and its derivatives

In the latest density functional study, we demonstrate that single-side-hydrogenated graphene (termed as

__) is a semiconductor with an indirect band gap of 1.35 eV, which is in between the gapless graph ene and wide band-gap graphane and surprisingly close to silicon. We show that its electronic structure and lattice characteristics are substantially different from those of graphene, graphone, or graphane. The lattice parameter and C-C bond length are found to be lengthened by 15% of those of graphene. Our binding-energy analysis confirms that such a single-sided hydrogenation leads to energetically stable material, making it a promising candidate as an organic semiconductor.__

*SSHGraphene**Phys. Rev. B 84, 041402(R) (2011) (Rapid Communications)*

Earlier, we have probed the transformation of graphene upon hydrogenation to graphane within the framework of density functional theory. By analysing the electronic structure for 18 different hydrogen concentrations, we brought out some novel features of this transition. Our results showed that the hydrogenation favored clustered configurations leading to the formation of compact islands. The analysis of the charge densities and electron localization functions (ELF) indicated that, as hydrogen coverage increases, the semi-metal turns into a metal, (showing a delocalized charge density,) then transforms into an insulator. The metallic phase was spatially inhomogeneous in the sense it contained islands of insulating regions formed by hydrogenated carbon atoms and metallic channels formed by contiguous bare carbon atoms. It turned out that it is possible to pattern the graphene sheet to tune the electronic structure. For example, removal of hydrogen atoms along the diagonal of the unit cell, yielding an armchair pattern at the edge, gave rise to a bandgap of 1.4 eV. We also showed that a weak ferromagnetic state exists even for a large hydrogen coverage whenever there was a sublattice imbalance in the presence of an odd number of hydrogen atoms.

*J. Phys.: Condens. Matter*

**22**465502,(2010)

*This work is featured in “IOP Select November-2010”*

In first problem we carried out ab initio electronic structure calculations on
*graphane* having single

### Proton hopping in Organometallic complex

Paper in communication

### Materials under extreme conditions

### Ab-initio studies of Ferroelectric materials

Lead zirconate titanate (PZT), a ceramic perovskite material is used as a component in ultra- sound transducers, ceramic capacitors, STM/AFM actuators, FRAM chips and sensors. PZT has also been used in the manufacture of ceramic resonators for reference timing in electronic circuitry. PZT is usually not used in its pure form but doped with either acceptors to create anion vacancies or with donors to create cation vacancies. Acceptor doping creates hard PZT and donor doping creates soft PZT, which are differentiated based on their piezoelectric constants. These piezoelectric constants are proportional to the polarization i.e. the electric field generated per unit of mechanical stress. Usually soft PZT has a higher piezoelectric constant but larger losses in material whereas hard PZT has lower piezoelectric constant and lower losses in material. With this view, we are interested in performing first principles density functional theory (DFT) based calculations on undoped and doped PZT materials.

### Solvation

We developed a technique to decrease memory requirements when solving the integral equations of three-dimensional (3D) molecular theory of solvation, a.k.a. 3D reference interaction site model (3D-RISM), using the modified direct inversion in the iterative subspace (MDIIS) numerical method of generalized minimal residual type. The latter provides robust convergence, in particular, for charged systems and electrolyte solutions with strong associative effects for which damped iterations do not converge. The MDIIS solver (typically, with 2 × 10 iterative vectors of argument and residual for fast convergence) treats the solute excluded volume (core), while handling the solvation shells in the 3D box with two vectors coupled with MDIIS iteratively and incorporating the electrostatic asymptotics outside the box analytically. For solvated systems from small to large macromolecules and solid–liquid interfaces, this results in 6- to 16-fold memory reduction and corresponding CPU load decrease in MDIIS. We illustrated the new technique on solvated systems of chemical and biomolecular relevance with different dimensionality, both in ambient water and aqueous electrolyte solution, by solving the 3D-RISM equations with the Kovalenko–Hirata (KH) closure, and the hypernetted chain (HNC) closure where convergent. This core–shell-asymptotics technique coupling MDIIS for the excluded volume core with iteration of the solvation shells converges as efficiently as MDIIS for the whole 3D box and yields the solvation structure and thermodynamics without loss of accuracy. Although being of benefit for solutes of any size, this memory reduction becomes critical in 3D-RISM calculations for large solvated systems, such as macromolecules in solution with ions, ligands, and other cofactors.

Journal of Computational Chemistry Volume 33, Issue 17, pages 1478–1494, 30 June 2012