01669nas a2200169   4500008004100000022001400041245012200055210006900177260012500246300001100371490000800382520091700390100002701307700001601334700002101350856012801371       2013                        eng d  a0021-960600aPerturbative approximations to single and double spin flip equation of motion coupled cluster singles doubles methods0 aPerturbative approximations to single and double spin flip equat  aCIRCULATION & FULFILLMENT DIV, 2 HUNTINGTON QUADRANGLE, STE 1 N O 1, MELVILLE, NY 11747-4501 USAbAMER INST PHYSICScSEP  a1241160 v1393 a<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>1 aDutta, Achintya, Kumar1 aPal, Sourav1 aGhosh, Debashree  uhttp://library.ncl.res.in/content/perturbative-approximations-single-and-double-spin-flip-equation-motion-coupled-cluster-001734nas a2200145   4500008004100000022001400041245011200055210006900167260008700236300001100323490000800334520110100342100002101443856012401464       2014                        eng d  a0021-960600aPerturbative approximation to hybrid equation of motion coupled cluster/effective fragment potential method0 aPerturbative approximation to hybrid equation of motion coupled   a1305 WALT WHITMAN RD, STE 300, MELVILLE, NY 11747-4501 USAbAMER INST PHYSICScMAR  a0941010 v1403 a<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>1 aGhosh, Debashree  uhttp://library.ncl.res.in/content/perturbative-approximation-hybrid-equation-motion-coupled-clustereffective-fragment-0