@article { ISI:000326355100005, title = {Performance of the EOMIP-CCSD(2) method for determining the structure and properties of doublet radicals: a benchmark investigation}, journal = {Journal of Chemical Theory and Computation}, volume = {9}, number = {10}, year = {2013}, month = {OCT}, pages = {4313-4331}, publisher = {AMER CHEMICAL SOC}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

We present a benchmark study on the performance of the EOMIP-CCSD(2) method for computation of structure and properties of doublet radicals. The EOMIP-CCSD(2) method is a second-order approximation to the standard EOMIP-CCSD method. By retaining the black box nature of the standard EOMIP-CCSD method and adding favorable N-5 scaling, the EOMIP-CCSD(2) method can become the method of choice for predicting the structure and spectroscopic properties of large doublet radicals. The EOMIP-CCSD(2) method overcomes the typical problems associated with the standard single reference ab initio treatment of doublet radicals. We compare our results for geometries and harmonic vibrational frequencies with those obtained using the standard EOMIP-CCSD method, as well as unrestricted Hartree-Fock (UHF)- and restricted open-shell Hartree-Fock (ROHF)-based single-reference coupled-cluster and second order many-body perturbation theory (MBPT(2)) methods. The effect of the basis set on the quality of the results has been studied using a hierarchy of Dunning{\textquoteright}s correlation-consistent aug-cc-pVXZ (X = D, T, Q) basis sets. Numerical results show that the EOMIP-CCSD(2) method, despite its N-5 scaling, gives better agreement with experimental results, compared to the UHF- and ROHF-based MBPT(2), as well as the single-reference coupled-cluster methods.

}, issn = {1549-9618}, doi = {10.1021/ct400316m}, author = {Dutta, Achintya Kumar and Vaval, Nayana and Pal, Sourav} } @article { ISI:000325392000050, title = {Perturbative approximations to single and double spin flip equation of motion coupled cluster singles doubles methods}, journal = {Journal of Chemical Physics}, volume = {139}, number = {12}, year = {2013}, month = {SEP}, pages = {124116}, publisher = {AMER INST PHYSICS}, address = {CIRCULATION \& FULFILLMENT DIV, 2 HUNTINGTON QUADRANGLE, STE 1 N O 1, MELVILLE, NY 11747-4501 USA}, abstract = {

Spin flip equation of motion coupled cluster (EOM-SF-CC) can correctly treat situations involving electronic degeneracies or near degeneracies, e. g., bond breaking, di- and tri-radicals, etc. However, for large systems EOM-SF-CC (even in single and double excitations) is computationally prohibitively expensive. Therefore, earlier approximations to EOM-SF-CC methods such as spin flip configuration interaction singles with perturbative doubles (SF-CIS(D)) have been proposed. In this work, we present a new perturbative approximation to EOM-SF-CC, which has been found to be more accurate than SF-CIS(D). The capabilities, advantages, and timings of the new approach have been demonstrated considering the singlet-triplet gaps in di- and triradicals as well as bond breaking examples. The method is extended to double spin flip EOM-CC and its capabilities have been tested. (C) 2013 AIP Publishing LLC.

}, issn = {0021-9606}, doi = {10.1063/1.4821936}, author = {Dutta, Achintya Kumar and Pal, Sourav and Ghosh, Debashree} } @article { ISI:000336020400012, title = {Partitioned EOMEA-MBPT(2): an efficient N-5 scaling method for calculation of electron affinities}, journal = {Journal of Chemical Theory and Computation}, volume = {10}, number = {5}, year = {2014}, month = {APR}, pages = {1923-1933}, publisher = {AMER CHEMICAL SOC}, type = {Article}, address = {1155 16TH ST, NW, WASHINGTON, DC 20036 USA}, abstract = {

We present an N-5 scaling modification to the standard EOMEA-CCSD method, based on the matrix partitioning technique and perturbative approximations. The method has lower computational scaling and smaller storage requirements than the standard EOMEA-CCSD method and, therefore, can be used to calculate electron affinities of large molecules and clusters. The performance and capabilities of the new method have been benchmarked with the standard EOMEA-CCSD method, for a test set of 20 small molecules, and the average absolute deviation is only 0.03 eV. The method is further used to investigate electron affinities of DNA and RNA nucleobases, and the results are in excellent agreement with the experimental values.

}, issn = {1549-9618}, doi = {10.1021/ct4009409}, author = {Dutta, Achintya Kumar and Gupta, Jitendra and Pathak, Himadri and Vaval, Nayana and Pal, Sourav} } @article { ISI:000357933400054, title = {Perturbative order analysis of the similarity transformed hamiltonian in fock-space coupled cluster theory: difference energy and electric response properties}, journal = {Molecular Physics}, volume = {113}, number = {13-14, SI}, year = {2015}, month = {JUL}, pages = {2046-2060}, publisher = {TAYLOR \& FRANCIS LTD}, address = {4 PARK SQUARE, MILTON PARK, ABINGDON OX14 4RN, OXON, ENGLAND}, abstract = {

A perturbative analysis of the ground-state similarity transformed Hamiltonian and its effect on the various Fock-space coupled cluster (FSCC) sectors is presented through calculation of ionisation potential, electron affinity, excitation energies and response properties. Various truncation schemes of the effective Hamiltonian are presented with explicit form of the defining equations. Based on such a truncation, the approximate methods are labelled as FSCC(n), where n represents the correlation energy of the ionised, electron attached or excited states corrected at least up to nth order within coupled cluster singles and doubles scheme (CCSD). A lower scaling CC2 type of approach (abbreviated as FS-CC2) is compared against the group of FSCC(n) methods for energies. Electric response properties have been compared and contrasted for the two lower scaling methods: FSCC(2) and FS-CC2. The various truncated methods are tested for a number of small molecules. The results obtained from a range of truncated methods are compared against full FSCCSD calculations.

}, keywords = {electric response properties, Fock space, perturbative order, similarity transformed Hamiltonian}, issn = {0026-8976}, doi = {10.1080/00268976.2015.1059509}, author = {Bhattacharya, Debarati and Dutta, Achintya Kumar and Gupta, Jitendra and Pal, Sourav} }