S Ramasesha
Articles written in Journal of Chemical Sciences
Volume 105 Issue 1 February 1993 pp 53-62 Physical and Theoretical
Is tetramethyleneethane a ground state triplet?
Aparna Chakrabarti I D L Albert S Ramasesha S Lalitha Jayaraman Chandrasekhar
We have examined a number of possible ways by which tetramethyleneethane (TME) can be a ground state triplet, as claimed by experimental studies, in violation of Ovchinnikov’s theorem for alternant hydrocarbons of equal bond lengths. Model exact π calculations of the low-lying states of TME, 3,4-dimethylenefuran and 3,4-dimethylenepyrrole were carried out using a diagrammatic valence bond approach. The calculations failed to yield a triplet ground state even after (a) tuning of electron correlation, (b) breaking alternancy symmetry, and (c) allowing for geometric distortions. In contrast to earlier studies of fine structure constants in other conjugated systems, the computed
Volume 106 Issue 2 April 1994 pp 433-443
Two-band extended Hubbard model studies show that the shift in optical gap of the metal-halogen (MX) chain upon embedding in a crystalline environment depends upon alternation in the site-diagonal electron-lattice interaction parameter (ε_{M}) and the strength of electron-electron interactions at the metal site (U_{M}). The equilibrium geometry studies on isolated chains show that the MX chains tend to distort for alternating ε_{M} and small
Volume 106 Issue 6 November 1994 pp 1261-1261
Volume 113 Issue 5-6 October 2001 pp 459-486
Quantum phenomena in magnetic nano clusters
C Raghu Indranil Rudra Diptiman Sen S Ramasesha
One of the fascinating fields of study in magnetism in recent years has been the study of quantum phenomena in nanosystems. While semiconductor structures have provided paradigms of nanosystems from the stand point of electronic phenomena, the synthesis of high nuclearity transition metal complexes have provided examples of nano magnets. The range and diversity of the properties exhibited by these systems rivals its electronic counterparts. Qualitative understanding of these phenomena requires only a knowledge of basic physics, but quantitative study throws up many challenges that are similar to those encountered in the study of correlated electronic systems. In this article, a brief overview of the current trends in this area are highlighted and some of the efforts of our group in developing a quantitative understanding of this field are outlined.
Volume 118 Issue 1 January 2006 pp 67-78
Symmetrized density-matrix-renormalization-group calculations have been carried out, within Pariser-Parr-Pople Hamiltonian, to e_{x}plore the nature of the ground and low-lying e_{x}cited states of long polythiophene oligomers. We have e_{x}ploited
Volume 121 Issue 5 September 2009 pp 823-837
Computing magnetic anisotropy constants of single molecule magnets
S Ramasesha Shaon Sahoo Rajamani Raghunathan Diptiman Sen
We present here a theoretical approach to compute the molecular magnetic anisotropy parameters, $D_M$ and $E_M$ for single molecule magnets in any given spin eigenstate of exchange spin Hamiltonian. We first describe a hybrid constant $M_S$-valence bond (VB) technique of solving spin Hamiltonians employing full spatial and spin symmetry adaptation and we illustrate this technique by solving the exchange Hamiltonian of the Cu_{6}Fe_{8} system. Treating the anisotropy Hamiltonian as perturbation, we compute the D$_M$ and E$_M$ values for various eigenstates of the exchange Hamiltonian. Since, the dipolar contribution to the magnetic anisotropy is negligibly small, we calculate the molecular anisotropy from the single-ion anisotropies of the metal centers. We have studied the variation of D$_M$ and E$_M$ by rotating the single-ion anisotropies in the case of Mn_{12}Ac and Fe_{8} SMMs in ground and few low-lying excited states of the exchange Hamiltonian. In both the systems, we find that the molecular anisotropy changes drastically when the single-ion anisotropies are rotated. While in Mn_{12}Ac SMM $D_M$ values depend strongly on the spin of the eigenstate, it is almost independent of the spin of the eigenstate in Fe_{8} SMM. We also find that the $D_M$ value is almost insensitive to the orientation of the anisotropy of the core Mn(IV) ions. The dependence of $D_M$ on the energy gap between the ground and the excited states in both the systems has also been studied by using different sets of exchange constants.
Volume 134, 2022
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