01533nas a2200229 4500008004100000022001400041245012200055210006900177260007300246300001000319490000800329520062400337653003700961653004700998653003601045653002201081100001701103700002001120700001601140700001801156856012901174 2012 eng d a0301-010400aConstrained variational approach for energy derivatives in Intermediate hamiltonian fock-space coupled-cluster theory0 aConstrained variational approach for energy derivatives in Inter aPO BOX 211, 1000 AE AMSTERDAM, NETHERLANDSbELSEVIER SCIENCE BVcJUN a45-490 v4013 a

Fock-space multi-reference coupled cluster theory is an efficient method for electronic structure of nearly degenerate cases. However, it suffers from the intruder state problem for large model space. The intermediate Hamiltonian formulation eliminates intruder state problem and helps in the convergence of equations. In this paper we have implemented intermediate Hamiltonian approach in Fock-space coupled cluster method for the response properties. We test our method for the dipole moments of doublet radicals. We report dipole moments of CN, SF and NS radicals. (C) 2011 Elsevier B. V. All rights reserved.

10aConstrained variational approach10aFock-space multi-reference coupled cluster10aIntermediate Hamiltonian theory10aResponse approach1 aGhosh, Aryya1 aGupta, Jitendra1 aPal, Sourav1 aVaval, Nayana uhttps://library.ncl.res.in/content/constrained-variational-approach-energy-derivatives-intermediate-hamiltonian-fock-space-001805nas a2200229 4500008004100000022001400041245011500055210006900170260007300239300001000312490000800322520095000330653001801280653002201298653002901320100002001349700002801369700001701397700001801414700001601432856012701448 2013 eng d a0301-010400aExtended coupled cluster through nth perturbation order for molecular response properties: a comparative study0 aExtended coupled cluster through nth perturbation order for mole aPO BOX 211, 1000 AE AMSTERDAM, NETHERLANDSbELSEVIER SCIENCE BVcMAY a45-510 v4173 a

In this paper, we have presented and analyzed the extended coupled cluster (ECC) energy functional, truncated up to nth perturbation order in which expansion is limit to operators of the single and double excitations. The expression for the first order energy derivatives based on the above truncation schemes has been derived and numerically analyzed. We have done a comparative study for dipole moment in various perturbative ECC (n) methods (2 <= n <= 5) for three systems (HF, H2O and BH) at equilibrium geometry. We have particularly studied two variants of ECC (4) and ECC (5*) method for stretched geometry. We have analyzed the contribution of important terms present in ECC functional based on perturbative argument. We have compared our results with standard coupled cluster (CCSD) as well as cubic truncated ECC and also benchmarked with the full CI results wherever available. (c) 2013 Elsevier B.V. All rights reserved.

10aDipole moment10aEnergy functional10aExtended coupled cluster1 aGupta, Jitendra1 aManohar, Prashant, Uday1 aGhosh, Aryya1 aVaval, Nayana1 aPal, Sourav uhttps://library.ncl.res.in/content/extended-coupled-cluster-through-nth-perturbation-order-molecular-response-properties-001546nas a2200169 4500008004100000022001400041245015900055210006900214260012500283300001100408490000800419520076700427100002001194700001801214700001601232856012801248 2013 eng d a0021-960600aLagrange multiplier approach for excited state properties through intermediate hamiltonian formulation of fock space multireference coupled-cluster theory0 aLagrange multiplier approach for excited state properties throug aCIRCULATION & FULFILLMENT DIV, 2 HUNTINGTON QUADRANGLE, STE 1 N O 1, MELVILLE, NY 11747-4501 USAbAMER INST PHYSICScAUG a0741080 v1393 a

In this paper, we present a formulation based on Lagrange multiplier approach for efficient evaluation of excited state energy derivatives in Fock space coupled cluster theory within the intermediate Hamiltonian framework. The formulation is applied to derive the explicit generic expressions up to second order energy derivatives for [1, 1] sector of Fock space with singles and doubles approximation. Its advantage, efficiency, and interconnection in comparison to the Lagrange multiplier approach in traditional formulation of Fock space, which is built on the concept of Bloch equation based effective Hamiltonian, has been discussed. Computational strategy for their implementation has also been discussed in some detail. (C) 2013 AIP Publishing LLC.

1 aGupta, Jitendra1 aVaval, Nayana1 aPal, Sourav uhttps://library.ncl.res.in/content/lagrange-multiplier-approach-excited-state-properties-through-intermediate-hamiltonian-001630nas a2200181 4500008004100000022001400041245010700055210006900162260007100231300001400302490000700316520092200323100002701245700002001272700001801292700001601310856012201326 2014 eng d a1549-961800aIntermediate hamiltonian fock space multireference coupled cluster approach to core excitation spectra0 aIntermediate hamiltonian fock space multireference coupled clust a1155 16TH ST, NW, WASHINGTON, DC 20036 USAbAMER CHEMICAL SOCcSEP a3656-36680 v103 a

The Fock space multireference coupled cluster (FSMRCC) method provides an efficient approach for the direct calculation of excitation energies. In intermediate Hamiltonian (IH-FSMRCC) formulation, the method is free from intruder state problems and associated convergence difficulties, even with a large model space. In this paper, we demonstrate that the IH-FSMRCC method with suitably chosen model space can be used for the accurate description of core excitation spectra of molecules, and our results are in excellent agreement with the experimental values. We have investigated the effect of choice of model space on the computed results. Unlike the equation-of-motion (EOM)-based method, the IH-FSMRCC does not require any special technique for convergence and in singles and doubles approximation gives a performance comparable to that of the standard EOMEE-CCSD method, even better in some of the cases.

1 aDutta, Achintya, Kumar1 aGupta, Jitendra1 aVaval, Nayana1 aPal, Sourav uhttps://library.ncl.res.in/content/intermediate-hamiltonian-fock-space-multireference-coupled-cluster-approach-core-001469nas a2200193 4500008004100000022001400041245010200055210006900157260007100226300001400297490000700311520073000318100002701048700002001075700002001095700001801115700001601133856012601149 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 uhttps://library.ncl.res.in/content/partitioned-eomea-mbpt2-efficient-n-5-scaling-method-calculation-electron-affinities-002044nas a2200229 4500008004100000022001400041245016300055210006900218260009200287300001400379490000800393520108200401653003301483653001501516653002301531653003901554100002701593700002701620700002001647700001601667856013101683 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 uhttps://library.ncl.res.in/content/perturbative-order-analysis-similarity-transformed-hamiltonian-fock-space-coupled-cluster-0