Sourav Pal

Ionization potential and structure relaxation of adenine, thymine, guanine and cytosine bases and their base pairs: A quantification of reactive sites

Journal of Molecular Structure: THEOCHEM, 2009

We present density functional theory (DFT) calculations using B3LYP/6-31++G ** method to show rel... more We present density functional theory (DFT) calculations using B3LYP/6-31++G ** method to show relaxation in geometry of base pairs on cation radical formation. The changes in hydrogen bond length and angles show that in the cationic radical form the structure of the base pairs relaxes due to the distribution of charge. According to a recent study, it has been found that, upon excitation hole transfer from base to sugar occurs which results in sugar radical formation and leads to strand breakage [45] [A. Kumar, M.D. Sevilla, J. Phys. Chem. B 110 (2006) 24181]. One hydrogen bond increases, while the other decreases in Adenine-Thymine (AT) base pair and in case of Guanine-Cytosine (GC) base pair, one bond increases and other two decrease. Same is the case with bond angles for both the base pairs. Analysis of the electron density map of Singly Occupied Molecular Orbital (SOMO) reveals that electron is transferred mainly from adenine and guanine bases in the cationic radical formation of AT and GC base pair, respectively. The reactive sites of bases have been analyzed using condensed Fukui functions in a relaxed and frozen core approximation. The effects of relaxation on the reactivity indices are also analyzed.

format_quoteCationic radical formation alters bond lengths and non-planarity in GC due to amino H-bonding effects.format_quote

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Relativistic extended-coupled-cluster method for the magnetic hyperfine structure constant

Physical Review A, 2015

The article deals with the general implementation of 4-component spinor relativistic extended cou... more The article deals with the general implementation of 4-component spinor relativistic extended coupled cluster (ECC) method to calculate first order property of atoms and molecules in their open-shell ground state configuration. The implemented relativistic ECC is employed to calculate hyperfine structure (HFS) constant of alkali metals (Li, Na, K, Rb and Cs), singly charged alkaline earth metal atoms (Be + , Mg + , Ca + and Sr +) and molecules (BeH, MgF and CaH). We have compared our ECC results with the calculations based on restricted active space configuration interaction (RAS-CI) method. Our results are in better agreement with the available experimental values than those of the RAS-CI values.

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Some aspects of self-consistent propagator theories

A general method for constructing bases for operator manifolds for any propagator, which satisfy ... more A general method for constructing bases for operator manifolds for any propagator, which satisfy "vacuum annihilation conditions" (VAC's) is developed. This approach is based on the observation that if the transformation of the unperturbed ground state to the correlated ground state is represented as a rotation in the Pock space, the corresponding rotation induced in the basis of the concerned operator space would generate a basis which satisfies VAC's on the correlated ground state. The associated requirements for the Hermiticity of superoperator Hamiltonian would also be met in this new basis. The proposed method is noniterative in that, once the form of the groundstate function is specified, the expansion of the operator manifold satisfying VAC s on the ground state does not require any iterative readjustment. The resultant propagators in this approach are fully linked. It is shown that this theory is equivalent to a propagator formalism in terms of holeand particle-creation and/or annihilation operators with a modified effective Harniltonian. The self-consistent electron and polarization propagators are considered as examples, and their underlying perturbative structures are analyzed. The role of density shift operators and higher-rank operators are discussed.

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Change of Hardness and Chemical Potential in Chemical Binding: A Quantitative Model

The Journal of Physical Chemistry, 1994

... Sourav Pal,' Ramkinkar Roy, and A. K. Chandra Physical Chemistry Division, National Chem... more

Extended Coupled Cluster Approach for Molecular Properties: Study of H2O and HF Complexes

International Journal of Molecular Sciences, 2001

In this paper, we study stationary variant of extended coupled-cluster response approach for prop... more In this paper, we study stationary variant of extended coupled-cluster response approach for properties. This has been studied at the singles and doubles approximation using cubic-truncated functional. This approximation has been studied earlier around equilibrium for small molecules. In this paper, efficacy of this approximation has been shown using perturbative arguments. Further we have calculated dipole moments and polarizabilities of weakly interacting dimers of HF, H 2 O and H 2 O-HF complex. Results of HF and H 2 O monomers have been presented at the same level for comparison. The results have been compared with experimental results, wherever available and other theoretical results.

format_quoteDouble-linking structure of the ECC functional ensures stable stationary equations, enabling effective calculations of molecular properties.format_quote

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Behaviour of density functional theory for electric response properties at distorted geometries of molecules

Theoretical Chemistry Accounts, 2012

The role of exchange-correlation is well known for accurate calculations of electric response pro... more The role of exchange-correlation is well known for accurate calculations of electric response properties. The exchange-correlation functional in density functional theory (DFT) has been well studied for ground state equilibrium geometry. However, the behaviour of these functionals in stretched geometries, where static correlation play an important role, has not been studied systematically, particularly for response electric properties. Thus we present here the rigorous calculation of electric response properties at distorted geometries of the molecules. We have considered dipole polarizability and dipole quadrupole polarizability for description of role of static and dynamic correlation for electric response properties. The calculations are performed with NIA-CPKS method described in earlier chapters. These DFT results are compared with higher level ab initio methods, such as CCSD and fully correlated full CI. We have studied single, double and triple bonded systems at different internuclear separation. We report here the dipole-polarizability and dipole-quadrupolepolarizability of HF, BH, H 2 CO, CO and NO +. We also present the effect of basis and functional on polarizability and dipole-quadrupole polarizability.

format_quoteThe reliability of dipole-quadrupole polarizabilities calculated with DFT was verified against high-level ab initio methods, confirming accuracy in electric response properties.format_quote

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Study of approximate coupled cluster methods for first-order static properties

Theoretica Chimica Acta, 1985

In this paper, we analyse the algebraic structure of the equations for calculating the first orde... more In this paper, we analyse the algebraic structure of the equations for calculating the first order static properties using several approximate versions of Coupled Cluster (CC) methods. In particular, the non-variational and the variational method using a CC wavefunction corresponding to an appropriately defined perturbed Hamiltonian as well as the simple expectation value expression using a CC stationary state are

Generalization of coupled-cluster response theory to multireference expansion spaces: application of the coupled-cluster singles and doubles effective Hamiltonian

Theoretical Chemistry Accounts: Theory, Computation, and Modeling (Theoretica Chimica Acta), 1999

Employing separate cluster ansatz in timeindependent and time-dependent wave-operators, coupled-c... more Employing separate cluster ansatz in timeindependent and time-dependent wave-operators, coupled-cluster (CC) response theory is generalized to multireference (MR) expansion spaces. For state energies, this corresponds to the MR secular problem with an arbitrary similarity-transformed eective Hamiltonian, r X À1 r X. The eective Hamiltonian can be generated via size-extensive CC methods. Thus the states in MR linear response theory (MRLRT) maintain the usual CC core-extensive properties. We have used the Gelfand unitary group basis of the spin-adapted con®gurations to construct the matrix ofr in the MR excitation space. As a preliminary application, the CC singles and doubles eective Hamiltonian is applied to excitation and photoionization energies of the CH and N 2 molecules, and is compared with experimental results and results from other numerical procedures including conventional CC linear response theory (CC-LRT), MR and full con®guration interaction (MRCI and FCI) methods. The numerical results indicate that MRLRT reproduces valence and external excited states quantitatively, combining the best features of CC-LRT and MRCI.

format_quoteMRLRT yields accurate excitation energies for one-electron states, achieving agreement with FCI results despite the absence of three-body interactions.format_quote

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A variational coupled cluster theory for closed shells using a propagator modification procedure

Theoretica Chimica Acta, 1985

A general partial summation method for including arbitrary classes of diagrams to all orders in t... more

On certain correspondences among various coupled-cluster theories for closed-shell systems

Pramana, 1982

In this paper certain correspondences have been shown among various formulations of coupled-clust... more In this paper certain correspondences have been shown among various formulations of coupled-cluster theories for many electron closed-shell systems. Specifically it is shown that (i) the energy functional using unitary ansatz of the form exp (T-T +) is exactly same order by order in T with the size-consistent energy functional ~ItKI~)/(~I~) recently obtained by us in coupled-cluster framework; (ii) in the framework of unitary ansatz of the form exp (T-T+), both non-variational and variational approaches lead to identical equations upto any given order in T and T + in T ~ T s approximation; (iii) variational procedure using our size-cousistent energy functional or using the functional obtained in the framework of unitary ansatz (as envisaged by Kutzelnigg) leads to energy in both cases, in T ... Tm approximation, for a total of quadratic powers in T and T +, same as Cizek's linearised coupled pair many electron theory energy; (iv)in case of practical calculation of the energy through the variational procedures using our size-consistent energy functional and the functional in unitary ansatz framework, there is a loss of upper bound.

format_quoteEstablishes correspondences among various energy functionals for closed-shell systems, highlighting notable features from the authors' studies.format_quote

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Analysis of coupled-cluster methods for higher-order static properties

Physical Review A, 1992

In this paper we present a useful analysis for higher-order static properties involving three dif... more

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Coupled-cluster response approach: Improved variational strategy

Physical Review A, 1990

In this paper we discuss an improved strategy for variational versions of the coupled-cluster res... more

Microwave Specific Wolff Rearrangement of α-Diazoketones and Its Relevance to the Nonthermal and Thermal Effect

The Journal of Organic Chemistry, 2002

Wolff rearrangement 1 of α-diazoketones is an integral part of the well-known Arndt−Eistert one-c... more

Bond length variations: Electron number profiles and transferable atomic sizes

Journal of Molecular Structure, 2009

A profile of the number of electrons with distance along the M-X bond in gas-phase diatomic molec... more A profile of the number of electrons with distance along the M-X bond in gas-phase diatomic molecules has been obtained from electron density plots calculated using DFT B3LYP 6-311G ** method for some representative molecules. This ''number profile" is compared with that expected from the partitioning of the 1D bond-distance into atom-specific transferable ''hub" or core atomic sizes of the M and X atoms and another ''axle" size which is associated with a pair of (bonding) electrons. The ''hub" size is proportional to a core atom-specific size, r nZ c with r nZ c (M) P r nZ c (X). For ''single bonds", the ''hub" size for M atom is C M r nZ c (M) and for X atom is C X r nZ c (X). The ''axle" size, D MX , is usually the ordinary ($4a H /3 where a H is the Bohr radius of the hydrogen atom) or elongated ($2a H) bond length of the hydrogen molecule. The ''hub" and ''axle" sizes could be characterized ''charge-transfer" (C M = p 2/3 = 2.144; C X = p 4/3 /2 = 2.300 and D MX = 4a H /3) or ''neutral" (C M or C X = 1, 2,. .. and D MX = 2a H). We use a new ''static" or ''peripatetic" classification for the core sizes which is derived from a new condition for metallization in elements based on atomic size. The charge-transfer distance, d MX ± , is usually found for ''static" conditions while the ''neutral" description is usually found when X = F or for ''peripatetic" conditions. Such a partitioning is seen to agree with that from the plot of the total number of electrons, N el , vs r along a bond axis. The N el vs r plots from each atom are described by a simple hydrogen-atom-like function which differ away (''out") or towards (''in") the M-X bond. Thus N in,out (M,X) = (Z M,X ± 1)exp (Àr/B in,out) where the minus sign is associated with M and plus sign with X and B in,out being related inversely to the Slater orbital exponent.

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Effect of Triples to Dipole Moments in Fock-Space Multireference Coupled Cluster Method

Journal of Chemical Theory and Computation, 2011

In this paper, we present the new implementation of partial triples for the dipole moment of doub... more In this paper, we present the new implementation of partial triples for the dipole moment of doublet radicals in Lagrangian formulation of Fock-space multireference coupled cluster (Λ-FSMRCC) response method. We have implemented a specific scheme of noniterative triples, in addition to singles and doubles schemes, which accounts for the effects appearing at least at the third order in dipole moments. The method is applied to the ground states of OH, OOH, HCOO, CN, CH, and PO radicals.

Density Functional Static Dipole Polarizability and First-Hyperpolarizability Calculations of Nan (n = 2, 4, 6, 8) Clusters Using an Approximate CPKS Method and its Comparison with MP2 Calculations

Journal of Chemical Theory and Computation, 2007

We report the static dipole polarizability and first-hyperpolarizability of the sodium atom clust... more We report the static dipole polarizability and first-hyperpolarizability of the sodium atom clusters, Nan, n = 2, 4, 6 and 8, using our recent implementation of a numerical-analytical approach to the coupled-perturbed Kohn-Sham (CPKS) equations in deMon2k. The calculations are reported for VWN and BP86 exchange-correlation functionals using Sadlej and TZVP-FIP1 basis sets which have been previously optimized for polarizability calculations. All-electron calculations were performed for the optimizations at the VWN/DZVP/A2 and PW86/DZVP/A2 levels. Comparisons are made with Hartree-Fock (HF) and MP2 benchmark calculations.

Separability of local reactivity descriptors

Journal of Chemical Sciences, 2005

The size-dependence of different local reactivity descriptors of dimer A 2 and AB type of systems... more The size-dependence of different local reactivity descriptors of dimer A 2 and AB type of systems is discussed. We derive analytic results of these descriptors calculated using finite difference approximation. In particular, we studied Fukui functions, relative electrophilicity and relative nucleophilicity, local softness and local philicity. The results are explained using the example of the dimer of BH 3 NH 3 .

format_quoteStudy of local reactivity descriptors in dimers examines extensive/intensive properties, utilizing BH3NH3 as a specific example.format_quote

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Multireference coupled cluster based analytic response approach for evaluating molecular properties: Some pilot results

The Journal of Chemical Physics, 1999

[The Journal of Chemical Physics 110, 2316 (1999)]. D. Ajitha, Nayana Vaval, Sourav Pal. Abstract... more

Size-extensive calculations of static structure factors from the coupled cluster singles and doubles model

The Journal of Chemical Physics, 1999

... Noboru Watanabe, Seiichiro Ten-no, Sourav Pal, Suehiro Iwata, Yasuo Udagawa. Abstract. ... (6... more

Fock space multireference coupled cluster calculations based on an underlying bivariational self-consistent field on Auger and shape resonances

The Journal of Chemical Physics, 2004

The Fock space multireference coupled cluster based on an underlying bivariational self-consisten... more The Fock space multireference coupled cluster based on an underlying bivariational self-consistent field is applied to the problem of computing complex energy associated with Auger and shape resonances in e-atom scattering. It is concluded that the Fock space multireference coupled cluster based on a bivariational self-consistent field provides a useful and practical approach to calculation of resonance parameters. Numerical results are presented for the 2 P shape resonance of Mg and Auger 1 s Ϫ1 hole of Be.

format_quoteThe FSMRCC method provides resonant parameters for 1s-1 Auger and 2P shape resonances, showing good agreement with experimental data.format_quote

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