@article { ISI:000306111900003, title = {Extended coupled cluster for Raman and infrared spectra of small molecules}, journal = {Chemical Physics}, volume = {403}, year = {2012}, month = {JUL}, pages = {25-32}, publisher = {ELSEVIER SCIENCE BV}, address = {PO BOX 211, 1000 AE AMSTERDAM, NETHERLANDS}, abstract = {

In this paper we study the harmonic vibrational frequencies, infrared (IR) intensities, Raman intensities and depolarization ratio using extended coupled cluster method. Raman and IR intensities are mixed derivatives of energy with respect to the electric field and geometric perturbation whereas vibrational frequencies are derivatives of energy with respect to geometry. We use semi-numerical approach to obtain these derivatives. We have studied the effect of electron correlation and basis set for the above properties. We compare our results with non-variational coupled cluster and experimental values, wherever available. We have studied HF, BH, CH+, CO and H2CO molecules in different basis sets. For HF molecule, benchmarking is done with full CI values and basis set convergence is studied for this molecule. Effect of triples is studied for all the molecules. (C) 2012 Elsevier B.V. All rights reserved.

}, keywords = {Extended coupled cluster, IR and Raman spectroscopy, Mixed derivatives}, issn = {0301-0104}, doi = {10.1016/j.chemphys.2012.04.017}, author = {Joshi, Sayali P. and Dutta, Achintya Kumar and Pal, Sourav and Vaval, Nayana} } @article { ISI:000345272700002, title = {Excited state geometry optimisation using fock-space multi-reference coupled cluster method}, journal = {Molecular Physics}, volume = {112}, number = {22}, year = {2014}, month = {NOV}, pages = {2884-2891}, publisher = {TAYLOR \& FRANCIS LTD}, type = {Article}, address = {4 PARK SQUARE, MILTON PARK, ABINGDON OX14 4RN, OXON, ENGLAND}, abstract = {

The Fock-space multi-reference coupled cluster method is used for the geometry optimisation of the low-lying excited states of the molecules. Molecular geometries of the carbon monohydride cation (CH)(+), water (H2O), ozone (O-3) and formaldehyde (HCHO) in their low-lying excited states are optimised. Excited state gradients are calculated using finite field multi-reference coupled cluster method. We compare our results with other theoretical and/or experimental results, wherever available.

}, keywords = {adiabatic excitation energy, excited state, Fock-space multi-reference coupled cluster, geometry optimisation, numerical derivative}, issn = {0026-8976}, doi = {10.1080/00268976.2014.915997}, author = {Dutta, Achintya Kumar and Bhattacharya, Sumantra} } @article { ISI:000354238700002, title = {Electron attachment to DNA and RNA nucleobases: an EOMCC investigation}, journal = {International Journal of Quantum Chemistry}, volume = {115}, number = {12}, year = {2015}, month = {JUN}, pages = {753-764}, publisher = {WILEY-BLACKWELL}, address = {111 RIVER ST, HOBOKEN 07030-5774, NJ USA}, abstract = {

We report a benchmark theoretical investigation of both vertical and adiabatic electron affinities of DNA and RNA nucleobases: adenine, guanine, cytosine, thymine, and uracil using equation of motion coupled cluster method. The vertical electron affinity (VEA) values of the first five states of the DNA and RNA nucleobases are computed. It is observed that the first electron attached state is energetically accessible in gas phase. Furthermore, an analysis of the natural orbitals exhibits that the first electron attached states of uracil and thymine are valence-bound in nature and undergo significant structural changes on attachment of an extra electron, which reflects in the deviation of the adiabatic electron affinity (AEA) than that of the vertical ones. Conversely, the first electron attached states of cytosine, adenine, and guanine are in the category of dipole-bound anions. Their structure, by and large, remain unaffected on attachment of an extra electron, which is evident from the observed small difference between the AEA and VEA values. VEA and AEA values of all the DNA and RNA nucleobases are found to be negative, which implies that the first electron attached states are not stable rather quasi bound. The results of all previous theoretical calculations are out of track and shows large deviation with respect to the experimentally measured values, whereas, our results are found to be in good agreement. Therefore, our computed values can be used as a reliable standard to calibrate new theoretical methods. (c) 2015 Wiley Periodicals, Inc.

}, keywords = {DNA-RNA bases, electron affinity, EOMCC}, issn = {0020-7608}, doi = {10.1002/qua.24892}, author = {Dutta, Achintya Kumar and Sengupta, Turbasu and Vaval, Nayana and Pal, Sourav} } @article { ISI:000356201700008, title = {EOMIP-CCSD(2)*: an efficient method for the calculation of ionization potentials}, journal = {Journal of Chemical Theory and Computation}, volume = {11}, number = {6}, year = {2015}, month = {JUN}, pages = {2461-2472}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

A new approximation within the domain of EOMIP-CC method is proposed. The proposed scheme is based on the perturbative truncation of the similarity transformed effective Hamiltonian matrix. We call it the EOMIP-CCSD(2)* method, which scales as noniterative N-6 and its storage requirement is very less, compared to the conventional EOMIP-CCSD method. The existing EOMIP-CCSD(2) method has a tendency to overestimate the ionization potential (IP) values. On the other hand, our new strategy corrects for the problem of such an overestimation, which is evident from the excellent agreement achieved with the experimental values. Furthermore, not only the ionization potential but also geometry and IR frequencies of problematic double radicals are estimated correctly, and the results are comparable to the CCSD(T) method, obviously at lesser computational cost. The EOMIP-CCSD(2)* method works even for the core ionization and satellite IP, where the earlier EOMIP-CCSD(2) approximation dramatically fails.

}, issn = {1549-9618}, doi = {10.1021/ct500927h}, author = {Dutta, Achintya Kumar and Vaval, Nayana and Pal, Sourav} } @article { ISI:000371103000013, title = {Electron detachment and subsequent structural changes of water clusters}, journal = {Journal of Physical Chemistry A}, volume = {120}, number = {7}, year = {2016}, month = {FEB}, pages = {1065-1073}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

A cost-effective equation of motion coupled cluster method, EOMIP-CCSD(2), is used to investigate vertical and adiabatic ionization potential as well as ionization-induced structural changes of water clusters and compared with CCSD(T), CASPT2, and MP2 methods. The moderate N-5 scaling and low storage requirement yields EOMIP-CCSD (2) calculation feasible even for reasonably large molecules and clusters with accuracy comparable to CCSD(T) method at much cheaper computational cost. Our calculations shed light on the authenticity of EOMIP-CCSD(2) results and establish a reliable method to study of ionization energy of molecular clusters. We have further investigated the performance of several classes of DFT functionals for ionization energies of water clusters to benchmark the results and to get a reliable functionals for the same.

}, issn = {1089-5639}, doi = {10.1021/acs.jpca.5b09389}, author = {Das, Susanta and Sengupta, Turbasu and Dutta, Achintya Kumar and Pal, Sourav} }