02064nas a2200169 4500008004100000022001400041245013600055210006900191260007100260300001400331490000600345520135700351100002701708700001801735700001601753856012501769 2013 eng d a1549-961800aPerformance of the EOMIP-CCSD(2) method for determining the structure and properties of doublet radicals: a benchmark investigation0 aPerformance of the EOMIPCCSD2 method for determining the structu a1155 16TH ST, NW, WASHINGTON, DC 20036 USAbAMER CHEMICAL SOCcOCT a4313-43310 v93 a

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'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.

1 aDutta, Achintya, Kumar1 aVaval, Nayana1 aPal, Sourav uhttp://library.ncl.res.in/content/performance-eomip-ccsd2-method-determining-structure-and-properties-doublet-radicals-001669nas 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

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.

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-001468nas a2200193 4500008004100000022001400041245010200055210006900157260007100226300001400297490000700311520073000318100002701048700002001075700002001095700001801115700001601133856012501149 2014 eng d a1549-961800aPartitioned EOMEA-MBPT(2): an efficient N-5 scaling method for calculation of electron affinities0 aPartitioned EOMEAMBPT2 an efficient N5 scaling method for calcul a1155 16TH ST, NW, WASHINGTON, DC 20036 USAbAMER CHEMICAL SOCcAPR a1923-19330 v103 a

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.

1 aDutta, Achintya, Kumar1 aGupta, Jitendra1 aPathak, Himadri1 aVaval, Nayana1 aPal, Sourav uhttp://library.ncl.res.in/content/partitioned-eomea-mbpt2-efficient-n-5-scaling-method-calculation-electron-affinities-002043nas a2200229 4500008004100000022001400041245016300055210006900218260009200287300001400379490000800393520108200401653003301483653001501516653002301531653003901554100002701593700002701620700002001647700001601667856013001683 2015 eng d a0026-897600aPerturbative order analysis of the similarity transformed hamiltonian in fock-space coupled cluster theory: difference energy and electric response properties0 aPerturbative order analysis of the similarity transformed hamilt a4 PARK SQUARE, MILTON PARK, ABINGDON OX14 4RN, OXON, ENGLANDbTAYLOR & FRANCIS LTDcJUL a2046-20600 v1133 a

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.

10aelectric response properties10aFock space10aperturbative order10asimilarity transformed Hamiltonian1 aBhattacharya, Debarati1 aDutta, Achintya, Kumar1 aGupta, Jitendra1 aPal, Sourav uhttp://library.ncl.res.in/content/perturbative-order-analysis-similarity-transformed-hamiltonian-fock-space-coupled-cluster-0