Subir Bhattacharyya

Bio: Subir Bhattacharyya is an academic researcher from University of Calcutta. The author has contributed to research in topics: Laser & Matrix (mathematics). The author has an hindex of 2, co-authored 3 publications receiving 56 citations.

A spin-adapted linear response theory in a coupled-cluster framework for direct calculation of spin-allowed and spin-forbidden transition energies

Abstract: In this paper, we have spin-adapted our recently formulated linear response theory in a coupled-cluster framework. This allows us to calculate directly both the spin-allowed and the spin-forbidden transition energies from a single methodology. We have introduced rank-zero and rank-one spin operators to construct excitation operators for singlet-singlet and singlet-triplet transitions respectively and utilised the graphical methods of spin algebra to integrate the spin variables. It has been shown how a suitable parameterisation of the reduced Hugenholtz matrix elements of the excitation operator in terms of Goldstone matrix elements makes the resulting system of equations simple, compact and suitable for computer implementation. A pilot calculation has been performed to test the applicability of the theory.

A linear response, coupled‐cluster theory for excitation energy

Abstract: Expressions for static and dynamic properties in coupled-cluster (CC) theory are derived. In the static case, using diagrammatic techniques, it is shown how consideration of orbital relaxation effects in the theory introduces higher-order correlation effects. For the dynamic case, excitation energy expressions are obtained without consideration of orbital relaxation effects and shown to be equivalent to an equation of motion (EOM) approach subject to a coupled-cluster ground-state wave function and an excitation operator consisting of single and double excitations. Illustrative applications for excited states of ethylene are reported.

Equation of motion coupled cluster method for electron attachment

Abstract: The electron attachment equation of motion coupled cluster (EA‐EOMCC) method is derived which enables determination of the various bound states of an (N+1)‐electron system and the corresponding energy eigenvalues relative to the energy of an N‐electron CCSD reference state Detailed working equations for the EA‐EOMCC method are derived using diagrammatic techniques for both closed‐shell and open‐shell CCSD reference states based upon a single determinant The EA‐EOMCC method is applied to a variety of different problems, the main purpose being to establish its prospects and limitations The results from EA‐EOMCC calculations are compared to other EOMCC approaches, starting from different reference states, as well as other theoretical methods and experimental values, where available We have investigated electron affinities for a wide selection of both closed‐shell and open‐shell systems Excitation spectra of atoms and molecules with an odd number of electrons are obtained, taking the closed‐shell ground state of the ion as a reference in the EA‐EOMCC calculation Finally we consider excitation spectra of some closed‐shell systems, and find in particular that the electron attachment approach is capable of yielding accurate triplet excitation energies in an efficient way