@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 = {<p>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.</p>},
	issn = {0021-9606},
	doi = {10.1063/1.4821936},
	author = {Dutta, Achintya Kumar and Pal, Sourav and Ghosh, Debashree}
}
@article { ISI:000334067400005,
	title = {Perturbative approximation to hybrid equation of motion coupled cluster/effective fragment potential method},
	journal = {Journal of Chemical Physics},
	volume = {140},
	number = {9},
	year = {2014},
	month = {MAR},
	pages = {094101},
	publisher = {AMER INST PHYSICS},
	address = {1305 WALT WHITMAN RD, STE 300, MELVILLE, NY 11747-4501 USA},
	abstract = {<p>Hybrid quantum mechanics/molecular mechanics (QM/MM) methods provide an attractive way to closely retain the accuracy of the QM method with the favorable computational scaling of the MM method. Therefore, it is not surprising that QM/MM methods are being increasingly used for large chemical/biological systems. Hybrid equation of motion coupled cluster singles doubles/effective fragment potential (EOM-CCSD/EFP) methods have been developed over the last few years to understand the effect of solvents and other condensed phases on the electronic spectra of chromophores. However, the computational cost of this approach is still dominated by the steep scaling of the EOM-CCSD method. In this work, we propose and implement perturbative approximations to the EOM-CCSD method in this hybrid scheme to reduce the cost of EOM-CCSD/EFP. The timings and accuracy of this hybrid approach is tested for calculation of ionization energies, excitation energies, and electron affinities of microsolvated nucleic acid bases (thymine and cytosine), phenol, and phenolate. (C) 2014 AIP Publishing LLC.</p>},
	issn = {0021-9606},
	doi = {10.1063/1.4866838},
	author = {Ghosh, Debashree}
}
