01176nas a2200205   4500008004100000022001400041245009800055210006900153260011000222300001400332490000700346520034900353100002200702700002200724700002700746700001600773700002400789700002800813856012900841       2012                        eng d  a1477-052000aConformational modulation of Ant-Pro oligomers using chirality alteration of proline residues0 aConformational modulation of AntPro oligomers using chirality al  aTHOMAS GRAHAM HOUSE, SCIENCE PARK, MILTON RD, CAMBRIDGE CB4 0WF, CAMBS, ENGLANDbROYAL SOC CHEMISTRYcSEP  a8426-84330 v103 a<p>Structural modulation of Ant-Pro (anthranilic acid-proline) oligomers has been carried out by chirality alteration of the proline residues. The results suggest that the chirality altered oligomers show well-defined helical conformation featuring nine-membered hydrogen bonding interactions - without compromising conformational rigidity.</p>1 aKale, Sangram, S.1 aKotmale, Amol, S.1 aDutta, Achintya, Kumar1 aPal, Sourav1 aRajamohanan, P., R.1 aSanjayan, Gangadhar, J.  uhttps://library.ncl.res.in/content/conformational-modulation-ant-pro-oligomers-using-chirality-alteration-proline-residues-001711nas a2200217   4500008004100000022001400041245007900055210006900134260007300203300001000276490000800286520092600294653002901220653003001249653002201279100002201301700002701323700001601350700001801366856010901384       2012                        eng d  a0301-010400aExtended coupled cluster for Raman and infrared spectra of small molecules0 aExtended coupled cluster for Raman and infrared spectra of small  aPO BOX 211, 1000 AE AMSTERDAM, NETHERLANDSbELSEVIER SCIENCE BVcJUL  a25-320 v4033 a<p>In this paper we study the harmonic vibrational frequencies, infrared (IR) intensities, Raman intensities and depolarization ratio using extended coupled cluster method. Raman and IR intensities are mixed derivatives of energy with respect to the electric field and geometric perturbation whereas vibrational frequencies are derivatives of energy with respect to geometry. We use semi-numerical approach to obtain these derivatives. We have studied the effect of electron correlation and basis set for the above properties. We compare our results with non-variational coupled cluster and experimental values, wherever available. We have studied HF, BH, CH+, CO and H2CO molecules in different basis sets. For HF molecule, benchmarking is done with full CI values and basis set convergence is studied for this molecule. Effect of triples is studied for all the molecules. (C) 2012 Elsevier B.V. All rights reserved.</p>10aExtended coupled cluster10aIR and Raman spectroscopy10aMixed derivatives1 aJoshi, Sayali, P.1 aDutta, Achintya, Kumar1 aPal, Sourav1 aVaval, Nayana  uhttps://library.ncl.res.in/content/extended-coupled-cluster-raman-and-infrared-spectra-small-molecules-001731nas a2200169   4500008004100000022001400041245009200055210006900147260007100216300001400287490000600301520107000307100002701377700001801404700001601422856012301438       2012                        eng d  a1549-961800aNOx catalyzed pathway of stratospheric ozone depletion: a coupled cluster investigation0 aNOx catalyzed pathway of stratospheric ozone depletion a coupled  a1155 16TH ST, NW, WASHINGTON, DC 20036 USAbAMER CHEMICAL SOCcMAY  a1895-19010 v83 a<p>We report a theoretical investigation on the NOx catalyzed pathways of stratospheric ozone depletion using highly accurate coupled cluster methods. These catalytic reactions represent a great challenge to state-of-the-art ab initio methods, while their mechanisms remain unclear to both experimentalists and theoreticians. In this work, we have used the so-called ``gold standard of quantum chemistry,'' the CCSD(T) method, to identify the saddle points on NOx-based reaction pathways of ozone hole formation. Energies of the saddle points are calculated using the multireference variants of coupled cluster methods. The calculated activation energies and rate constants show good agreement with available experimental results. Tropospheric precursors to stratospheric NOx radicals have been identified, and their potential importance in stratospheric chemistry has been discussed. Our calculations resolve previous conflicts between ab initio and experimental results for a trans nitro peroxide intermediate, in the NOx catalyzed pathway of ozone depletion.</p>1 aDutta, Achintya, Kumar1 aVaval, Nayana1 aPal, Sourav  uhttps://library.ncl.res.in/content/nox-catalyzed-pathway-stratospheric-ozone-depletion-coupled-cluster-investigation-001284nas a2200193   4500008004100000022001400041245022000055210006900275260011000344300001600454490000600470520037000476100001900846700003100865700002700896700001900923700001600942856013200958       2013                        eng d  a2046-206900aHighly concise and practical route to clavaminols, sphinganine and (+)-spisulosine via indium mediated allylation of alpha-hydrazino aldehyde and a theoretical insight into the stereochemical aspects of the reaction0 aHighly concise and practical route to clavaminols sphinganine an  aTHOMAS GRAHAM HOUSE, SCIENCE PARK, MILTON RD, CAMBRIDGE CB4 0WF, CAMBS, ENGLANDbROYAL SOC CHEMISTRYcJUL  a15442-154480 v33 a<p>A conceptually different approach has been employed for the synthesis of 1,2-amino alcohols by proline-catalyzed alpha-amination of aldehyde and one-pot indium mediated allylation of the crude alpha-hydrazino aldehydes. DFT based quantum chemical calculations have been performed to obtain a quantitative explanation of the stereoselectivity of the reaction.</p>1 aPandey, Menaka1 aChowdhury, Partha, Sarathi1 aDutta, Achintya, Kumar1 aKumar, Pradeep1 aPal, Sourav  uhttps://library.ncl.res.in/content/highly-concise-and-practical-route-clavaminols-sphinganine-and-spisulosine-indium-mediated-002065nas a2200169   4500008004100000022001400041245013600055210006900191260007100260300001400331490000600345520135700351100002701708700001801735700001601753856012601769       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<p>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.</p>1 aDutta, Achintya, Kumar1 aVaval, Nayana1 aPal, Sourav  uhttps://library.ncl.res.in/content/performance-eomip-ccsd2-method-determining-structure-and-properties-doublet-radicals-001670nas a2200169   4500008004100000022001400041245012200055210006900177260012500246300001100371490000800382520091700390100002701307700001601334700002101350856012901371       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  uhttps://library.ncl.res.in/content/perturbative-approximations-single-and-double-spin-flip-equation-motion-coupled-cluster-001387nas a2200217   4500008004100000022001400041245009600055210006900151260009200220300001400312490000800326520050700334653003200841653001800873653004700891653002600938653002500964100002700989700002701016856012601043       2014                        eng d  a0026-897600aExcited state geometry optimisation using fock-space multi-reference coupled cluster method0 aExcited state geometry optimisation using fockspace multireferen  a4 PARK SQUARE, MILTON PARK, ABINGDON OX14 4RN, OXON, ENGLANDbTAYLOR & FRANCIS LTDcNOV  a2884-28910 v1123 a<p>The Fock-space multi-reference coupled cluster method is used for the geometry optimisation of the low-lying excited states of the molecules. Molecular geometries of the carbon monohydride cation (CH)(+), water (H2O), ozone (O-3) and formaldehyde (HCHO) in their low-lying excited states are optimised. Excited state gradients are calculated using finite field multi-reference coupled cluster method. We compare our results with other theoretical and/or experimental results, wherever available.</p>10aadiabatic excitation energy10aexcited state10aFock-space multi-reference coupled cluster10ageometry optimisation10anumerical derivative1 aDutta, Achintya, Kumar1 aBhattacharya, Sumantra  uhttps://library.ncl.res.in/content/excited-state-geometry-optimisation-using-fock-space-multi-reference-coupled-cluster-001513nas a2200181   4500008004100000022001400041245005700055210005500112260008700167300001100254490000800265520088000273100002701153700002801180700001801208700001601226856008901242       2014                        eng d  a0021-960600aGround state of naphthyl cation: singlet or triplet?0 aGround state of naphthyl cation singlet or triplet  a1305 WALT WHITMAN RD, STE 300, MELVILLE, NY 11747-4501 USAbAMER INST PHYSICScMAR  a1143120 v1403 a<p>We present a benchmark theoretical investigation on the electronic structure and singlet- triplet(ST) gap of 1- and 2-naphthyl cations using the CCSD(T) method. Our calculations reveal that the ground states of both the naphthyl cations are singlet, contrary to the results obtained by DFT/B3LYP calculations reported in previous theoretical studies. However, the triplet states obtained in the two structural isomers of naphthyl cation are completely different. The triplet state in 1-naphthyl cation is (pi,sigma) type, whereas in 2-naphthyl cation it is (pi,sigma) type, The S-T gaps in naphthyl cations and the relative stability ordering of the singlet and the triplet states are highly sensitive to the basis set quality as well as level of correlation, and demand for inclusion of perturbative triples in the coupled-cluster ansatz. (c) 2014 AIP Publishing LLC.</p>1 aDutta, Achintya, Kumar1 aManohar, Prashant, Uday1 aVaval, Nayana1 aPal, Sourav  uhttps://library.ncl.res.in/content/ground-state-naphthyl-cation-singlet-or-triplet-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<p>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.</p>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<p>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.</p>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-001852nas a2200181   4500008004100000022001400041245011900055210006900174260007100243300001400314490000800328520112900336100002701465700001701492700001801509700001601527856012701543       2014                        eng d  a1089-563900aStructure, stability, and properties of the trans peroxo nitrate radical: the importance of nondynamic correlation0 aStructure stability and properties of the trans peroxo nitrate r  a1155 16TH ST, NW, WASHINGTON, DC 20036 USAbAMER CHEMICAL SOCcFEB  a1350-13620 v1183 a<p>We report a comparative single-reference and multireference coupled-cluster investigation on the structure, potential energy surface, and IR spectroscopic properties of the trans peroxo nitrate radical, one of the key intermediates in stratospheric NOx chemistry. The previous single-reference ab initio studies predicted an unbound structure for the trans peroxo nitrate radical. However, our Fock space multireference coupled-cluster calculation confirms a bound structure for the trans peroxo nitrate radical, in accordance with the experimental results reported earlier. Further, the analysis of the potential energy surface in FSMRCC method indicates a well-behaved minima, contrary to the shallow minima predicted by the single-reference coupled-cluster method. The harmonic force field analysis, of various possible isomers of peroxo nitrate also reveals that only the trans structure leads to the experimentally observed IR peak at 1840 cm(-1). The present study highlights the critical importance of nondynamic correlation in predicting the structure and properties of high-energy stratospheric NOx radicals.</p>1 aDutta, Achintya, Kumar1 aDar, Manzoor1 aVaval, Nayana1 aPal, Sourav  uhttps://library.ncl.res.in/content/structure-stability-and-properties-trans-peroxo-nitrate-radical-importance-nondynamic-002323nas a2200217   4500008004100000022001400041245007500055210006900130260006700199300001200266490000800278520158100286653001801867653002201885653001001907100002701917700002201944700001801966700001601984856010502000       2015                        eng d  a0020-760800aElectron attachment to DNA and RNA nucleobases: an EOMCC investigation0 aElectron attachment to DNA and RNA nucleobases an EOMCC investig  a111 RIVER ST, HOBOKEN 07030-5774, NJ USAbWILEY-BLACKWELLcJUN  a753-7640 v1153 a<p>We report a benchmark theoretical investigation of both vertical and adiabatic electron affinities of DNA and RNA nucleobases: adenine, guanine, cytosine, thymine, and uracil using equation of motion coupled cluster method. The vertical electron affinity (VEA) values of the first five states of the DNA and RNA nucleobases are computed. It is observed that the first electron attached state is energetically accessible in gas phase. Furthermore, an analysis of the natural orbitals exhibits that the first electron attached states of uracil and thymine are valence-bound in nature and undergo significant structural changes on attachment of an extra electron, which reflects in the deviation of the adiabatic electron affinity (AEA) than that of the vertical ones. Conversely, the first electron attached states of cytosine, adenine, and guanine are in the category of dipole-bound anions. Their structure, by and large, remain unaffected on attachment of an extra electron, which is evident from the observed small difference between the AEA and VEA values. VEA and AEA values of all the DNA and RNA nucleobases are found to be negative, which implies that the first electron attached states are not stable rather quasi bound. The results of all previous theoretical calculations are out of track and shows large deviation with respect to the experimentally measured values, whereas, our results are found to be in good agreement. Therefore, our computed values can be used as a reliable standard to calibrate new theoretical methods. (c) 2015 Wiley Periodicals, Inc.</p>10aDNA-RNA bases10aelectron affinity10aEOMCC1 aDutta, Achintya, Kumar1 aSengupta, Turbasu1 aVaval, Nayana1 aPal, Sourav  uhttps://library.ncl.res.in/content/electron-attachment-dna-and-rna-nucleobases-eomcc-investigation-001653nas a2200169   4500008004100000022001400041245008500055210006900140260007100209300001400280490000700294520101700301100002701318700001801345700001601363856010401379       2015                        eng d  a1549-961800aEOMIP-CCSD(2)*: an efficient method for the calculation of ionization potentials0 aEOMIPCCSD2 an efficient method for the calculation of ionization  a1155 16TH ST, NW, WASHINGTON, DC 20036 USAbAMER CHEMICAL SOCcJUN  a2461-24720 v113 a<p>A new approximation within the domain of EOMIP-CC method is proposed. The proposed scheme is based on the perturbative truncation of the similarity transformed effective Hamiltonian matrix. We call it the EOMIP-CCSD(2)* method, which scales as noniterative N-6 and its storage requirement is very less, compared to the conventional EOMIP-CCSD method. The existing EOMIP-CCSD(2) method has a tendency to overestimate the ionization potential (IP) values. On the other hand, our new strategy corrects for the problem of such an overestimation, which is evident from the excellent agreement achieved with the experimental values. Furthermore, not only the ionization potential but also geometry and IR frequencies of problematic double radicals are estimated correctly, and the results are comparable to the CCSD(T) method, obviously at lesser computational cost. The EOMIP-CCSD(2)* method works even for the core ionization and satellite IP, where the earlier EOMIP-CCSD(2) approximation dramatically fails.</p>1 aDutta, Achintya, Kumar1 aVaval, Nayana1 aPal, Sourav  uhttps://library.ncl.res.in/content/eomip-ccsd2-efficient-method-calculation-ionization-potentials-101795nas a2200169   4500008004100000022001400041245015800055210006900213260008700282300002300369490000800392520103500400100002701435700001801462700001601480856012901496       2015                        eng d  a0021-960600aNew scheme for perturbative triples correction to (0,1) sector of Fock space multi-reference coupled cluster method: Theory, implementation, and examples0 aNew scheme for perturbative triples correction to 01 sector of F  a1305 WALT WHITMAN RD, STE 300, MELVILLE, NY 11747-4501 USAbAMER INST PHYSICScJAN  aArticle No. 0441130 v1423 a<p>We propose a new elegant strategy to implement third order triples correction in the light of many-body perturbation theory to the Fock space multi-reference coupled cluster method for the ionization problem. The computational scaling as well as the storage requirement is of key concerns in any many-body calculations. Our proposed approach scales as N-6 does not require the storage of triples amplitudes and gives superior agreement over all the previous attempts made. This approach is capable of calculating multiple roots in a single calculation in contrast to the inclusion of perturbative triples in the equation of motion variant of the coupled cluster theory, where each root needs to be computed in a state-specific way and requires both the left and right state vectors together. The performance of the newly implemented scheme is tested by applying to methylene, boron nitride (B2N) anion, nitrogen, water, carbon monoxide, acetylene, formaldehyde, and thymine monomer, a DNA base. (C) 2015 AIP Publishing LLC.</p>1 aDutta, Achintya, Kumar1 aVaval, Nayana1 aPal, Sourav  uhttps://library.ncl.res.in/content/new-scheme-perturbative-triples-correction-01-sector-fock-space-multi-reference-coupled-102044nas 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<p>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.</p>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-001519nas a2200181   4500008004100000022001400041245007600055210006900131260007100200300001400271490000800285520085200293100001701145700002201162700002701184700001601211856011001227       2016                        eng d  a1089-563900aElectron detachment and subsequent structural changes of water clusters0 aElectron detachment and subsequent structural changes of water c  a1155 16TH ST, NW, WASHINGTON, DC 20036 USAbAMER CHEMICAL SOCcFEB  a1065-10730 v1203 a<p>A cost-effective equation of motion coupled cluster method, EOMIP-CCSD(2), is used to investigate vertical and adiabatic ionization potential as well as ionization-induced structural changes of water clusters and compared with CCSD(T), CASPT2, and MP2 methods. The moderate N-5 scaling and low storage requirement yields EOMIP-CCSD (2) calculation feasible even for reasonably large molecules and clusters with accuracy comparable to CCSD(T) method at much cheaper computational cost. Our calculations shed light on the authenticity of EOMIP-CCSD(2) results and establish a reliable method to study of ionization energy of molecular clusters. We have further investigated the performance of several classes of DFT functionals for ionization energies of water clusters to benchmark the results and to get a reliable functionals for the same.</p>1 aDas, Susanta1 aSengupta, Turbasu1 aDutta, Achintya, Kumar1 aPal, Sourav  uhttps://library.ncl.res.in/content/electron-detachment-and-subsequent-structural-changes-water-clusters-001979nas a2200373   4500008004100000022001400041245012500055210006900180260000800249300001400257490000800271520089200279653000701171653000701178653002701185653000701212653000701219653002401226653002501250653000801275653002301283653001801306653001501324653000701339653000801346653002701354653000701381653000701388653001301395100001801408700002701426700002801453856012401481       2017                        eng d  a0974-362600aResolution of the identity and cholesky representation of EOM-MP2 approximation: implementation, accuracy and efficiency0 aResolution of the identity and cholesky representation of EOMMP2  cOCT  a1611-16260 v1293 aWe present a Resolution of Identity and Cholesky Decomposition Based Implementation of EOM-MP2 approximation. The RI and CD based EOM-MP2 shows significant speed-up and less storage requirement than the conventional canonical version and can be applied to very large systems. The new algorithm used for this implementation eliminates the most storage requiring four-index quantities resulting in the decrease of storage requirement, reduction in I/O penalties and improved parallel performance, at the expense of more floating point operations. Therefore, the speed-up compared to conventional EOM-MP2 method is more prominent in case of EA, EE and SF case where the storage bottleneck is significant than the EOM-IP-MP2 method, where the storage requirement is significantly less. However, the RI/CD based EOM-IP-MP2 can be coupled with frozen natural orbitals to gain further speed-up.10aCC10aCD10aCholesky decomposition10aEA10aEE10aelectron attachment10aelectron correlation10aEOM10aequation-of-motion10aexcited state10aionization10aIP10aMP210aResolution-of-identity10aRI10aSF10aspinflip1 aKumar, Dinesh1 aDutta, Achintya, Kumar1 aManohar, Prashant, Uday  uhttps://library.ncl.res.in/content/resolution-identity-and-cholesky-representation-eom-mp2-approximation-implementation01372nas a2200205   4500008004100000022001400041245009300055210006900148260000800217300001100225490000800236520069000244653001300934653002500947653001800972100002700990700001801017700001601035856011501051       2018                        eng d  a0020-760800aLower scaling approximation to EOM-CCSD: a critical assessment of the ionization problem0 aLower scaling approximation to EOMCCSD a critical assessment of   cJUL  ae255940 v1183 a<p>In this article, we investigate the performance of different approximate variants of the EOM-CCSD method for calculation of ionization potential (IP), as compared to EOM-CCSDT reference values. None of the lower scaling approximations to the EOM-CCSD method give a consistent performance for valence, inner valence, and core ionization, favoring one, or the other depending on the nature of the approximation used. The parent EOMIP-CCSD method gives superior performance for valence IP but can show large errors for inner valence and core ionization. The problem is particularly severe for core-ionization, where even the EOMIP-CCSDT method cannot provide quantitative accuracy.</p>10aEOM-CCSD10aIonization potential10alower scaling1 aDutta, Achintya, Kumar1 aVaval, Nayana1 aPal, Sourav  uhttps://library.ncl.res.in/content/lower-scaling-approximation-eom-ccsd-critical-assessment-ionization-problem