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Featured researches published by Kalyan K. Das.


Chemical Physics Letters | 1986

On the direct determination of constrained pure state one-electron density matrices: a new algorithm

Kalyan K. Das; S.P. Bhattacharyya

Abstract An algorithm based on a “constrained variational principle” is suggested for the direct determination of constrained one-electron density matrices in a self-consistent manner. The algorithm uses both penalty-function and Lagrangian multiplier methods of incorporating equality constraints and can tackle any number of constraints. Results of preliminary calculations are presented.


International Journal of Quantum Chemistry | 1996

A spin-adapted coupled-cluster based linear response theory for double ionization potentials

Rajat K. Chaudhuri; Barnali Datta; Kalyan K. Das; Debashis Mukherjee

We developed in this article a spin-adapted formulation of the coupled-cluster based linear response theory (CC-LRT) for computing double-ionization potentials (DIPs), which may be experimentally observed by Auger spectroscopy. CC-LRT is a multireference generalization of the CC theory where the energy differences have no disconnected vacuum (core) diagrams, signifying core-extensivity. For the spin-adaptation of the CC-LRT equations for the singlet and triplet manifolds, we used the Young-Yamanouchi orthogonal spin-eigenfunctions. The orbital version of the CC-LRT equations are then automatically generated by the conjugate projection operators of Young-Yamanouchi spin functions. We illustrated the working of our spin-adaptation procedure by confining our CC-LRT equations to the space of 2h and 1p–3h ionized determinants. As numerical application of our formalism, we computed the Auger kinetic energies of HF and H2O. We also analyzed the nature of size-extensivity of the DIPs generated by CC-LRT and showed explicitly that when the molecule is composed of two noninteracting fragments the computed DIPs are either DIPs of fragment A or B or a composite DIP depending on both A and B, which are just not sum of ionization potentials (IPs) of A and B. This analysis is done to underscore the fact that DIPs from CC-LRT is only core-extensive and not fully extensive.


Pramana | 1985

On the direct determination of constrained pure state one-electron density matrices: Part I. A new theoretical model for closed-shell systems

Kalyan K. Das; Priyotosh Khan; Sankar Prasad Bhattacharyya

A new method based on the penalty-function way of satisfying equality constraints is proposed for the determination of constrained pure state one-electron density matrices for closed-shell many-electron systems. The algorithm suggested can handle many constraints simultaneously. Certain interesting features of the proposed algorithm are discussed with numerical examples.


Pramana | 1987

On the direct determination of constrained pure state one-electron density matrices: II. A modified algorithm and its applications

Kalyan K. Das; Priyotosh Khan; Sankar Prasad Bhattacharyya

An algorithm recently proposed by us for the direct determination of pure state one-electron density matrices (P) under externally imposed constraints has been remodelled. The modified algorithm is applied to the construction of ground state potential energy curve of lithium hydride molecule usingknown values of dipole moments of LiH at various internuclear distances as theexternal constraint. The equilibrium internuclear distances (Rc) is seen to be unaffected by the constraint, but the force constant improves remarkably. The relevant features of the constrained density vis-a-vis those of the unconstrained one are analyzed revealing some of the improved features of the constrained density.


Pramana | 1993

Handling of intrinsic divergence in the orthogonal gradient method of orbital optimization in an MC-SCF framework and geometry optimization in pathological cases: I (propynal in excited states)

C Medhi; S.P. Bhattacharyya; Kalyan K. Das

It is demonstrated that a generalized version of the orthogonal gradient method of orbital optimization may sometimes encounter a specific divergence problem which may be termed intrinsic to the first order method. Instead of switching over to a more sophisticated second order method one can cure the divergence problem at the first order level itself by suitably tailoring the MC-SCF operator or the MC-SCF energy matrix. Results of complete geometry optimization of propynal inl,3nπ* and3ππ* states (pathological cases) are reported to demonstrate the usefulness of the method at an INDO-MCSCF level of approximation. The results of structure calculations are further rationalized from generalized quantum chemical bond order indices.


Journal of Chemical Sciences | 1989

On the usefulness of generalised quantum chemical valence parameters in monitoring the course of a chemical reaction: a case study of the photochemical decomposition of HNO in excited states

D. K. Maity; Kalyan K. Das; S.P. Bhattacharyya

A variant of the orthogonal gradient method of orbital optimization in the INDO-MCSCF framework has been used to study the photochemical decomposition of the HNO molecule into H + NO in the lowest1.3A″ states. A complete geometry optimization has been carried out at all points of the reaction path which appears to be almost barrierless. The one-electron density matrix extracted from the optimized wavefunction at each point has been used to generate the relevant sets of quantum chemical valence parameters. A sharp transition is noted in the N-H bond order and hydrogen free valence index when plotted as functions of rNH. This enables us to locate the transition region easily.


Journal of Chemical Sciences | 1988

Cation-inert gas atom interactions: A look into charge transfer energetics

Kalyan K. Das; S.P. Bhattacharyya

The charge-transfer energetics of interactions in a series of closed-shell cation-inert gas pairs is studied by using a model based on the electronegativity equalization principle. These results arc compared with those obtained from SCF calculations carried out at the STO-3G level. A model interaction potential is tested for these systems. The possible effect of an additional electrostatic factor in the charge transfer process is investigated.


Journal of Chemical Sciences | 1987

On the use of dipole moment as a collective coordinate in constrained variational calculations

Priyotosh Khan; Kalyan K. Das; S.P. Bhattacharyya

The dipole moment function of a diatomic molecule may be viewed as a continuous collective coordinate which encodes a lot of information about sharing of electrons between the atoms concerned. By its very nature it takes cognizance of certain many-body effects and should shape the constrained wavefunction or the one electron density in much the same way as would explicit inclusion of the same many-body effects. A case study of the problem with lithium hydride as the model system has been presented and the long range behaviour of the constrained density analysed. The spectrum of the constrained Fock operator is compared with that of the unconstrained one.


Journal of Chemical Sciences | 1986

The scaled one-electron Hamiltonian model for open-shelllcao-mo-scf calculations: further corrections to thesoeh energy

Kalyan K. Das; Sankar Prasad

The wavefunction generated by the scaled one-electron Hamiltonian (soeh) model has been modified further by including all single excitations from the space of thesoeh function. A perturbation-variation ansatz has been invoked for obtaining the corresponding energy correction (ΔE2). [E(soeh)+ΔE2] is shown to reproduceE(Roothaan) very closely. It has been demonstrated that by making ΔE2 stationary with respect to change in μ results. The correctedsoeh [E(soeh)+ΔE2] is shown to be quite useful for the calculation of geometrical parameters of open-shell systems even though it lacks the ‘upper-boundedness’ exhibited byE(soeh).


International Journal of Quantum Chemistry | 1989

Spectroscopic and structural features of small thiocarbonyl molecules in low-lying excited states: Further applications of a variant of the orthogonal gradient method

Kalyan K. Das; Debashis Mukherjee; S.P. Bhattacharyya

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S.P. Bhattacharyya

Indian Association for the Cultivation of Science

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Debashis Mukherjee

Indian Association for the Cultivation of Science

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Priyotosh Khan

Indian Association for the Cultivation of Science

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Sankar Prasad Bhattacharyya

Indian Association for the Cultivation of Science

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Barnali Datta

Indian Association for the Cultivation of Science

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C Medhi

Indian Association for the Cultivation of Science

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D. K. Maity

Indian Association for the Cultivation of Science

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Debendra Madhab Bhattacharyya

Indian Association for the Cultivation of Science

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Rajat K. Chaudhuri

Indian Institute of Astrophysics

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Sankar Prasad

Indian Association for the Cultivation of Science

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