Chemistry transport models (CTMs) play an important role in understanding fluxes and atmospheric distribution of carbon dioxide (CO_{2}). They have been widely used for modeling CO_{2} transport through forward simulations and inferring fluxes through inversion systems. With the increasing availability of high-resolution observations, it has been become possible to estimate CO_{2} fluxes at higher spatial resolution. In this work, we implemented CO_{2} transport in the Model for Prediction Across Scales – Atmosphere (MPAS-A). The objective is to use the variable-resolution capability of MPAS-A to enable a high-resolution CO_{2} simulation in a limited region with a global model. Treating CO_{2} as an inert tracer, we implemented in MPAS-A (v6.3) the CO_{2} transport processes, including advection, vertical mixing by boundary layer scheme, and convective transport. We first evaluated the newly implemented model's tracer mass conservation and then its CO_{2} simulation accuracy. A 1-year (2014) MPAS-A simulation is evaluated at the global scale using CO_{2} measurements from 50 near-surface stations and 18 Total Carbon Column Observing Network (TCCON) stations. The simulation is also compared with two global models: National Oceanic and Atmospheric Administration (NOAA) CarbonTracker v2019 (CT2019) and European Centre for Medium-Range Weather Forecasts (ECMWF) Integrated Forecasting System (IFS). A second set of simulation (2016–2018) is used to evaluate MPAS-A at regional scale using Atmospheric Carbon and Transport – America (ACT-America) aircraft CO_{2} measurements over the eastern United States. This simulation is also compared with CT2019 and a 27 km WRF-Chem simulation. The global-scale evaluations show that MPAS-A is capable of representing the spatial and temporal CO_{2} variation with a comparable level of accuracy as IFS of similar horizontal resolution. The regional-scale evaluations show that MPAS-A is capable of representing the observed atmospheric CO_{2} spatial structures related to the midlatitude synoptic weather system, including the warm versus cold sector distinction, boundary layer to free troposphere difference, and frontal boundary CO_{2} enhancement. MPAS-A's performance in representing these CO_{2} spatial structures is comparable to the global model CT2019 and regional model WRF-Chem.

Influenza fusion peptide is critical for mediating the fusion of viral and host cell membranes during viral entry. The interaction of monomeric influenza fusion peptide with membranes is studied with replica exchange molecular dynamics simulations using a new implicit membrane model to effectively reach microsecond to millisecond time scales. The conformational sampling of the fusion peptide was studied as a function of different N- and C-termini, including an experimental construct with an additional C-terminal tag, as well as a function of protonation of acidic residues. It is found that the influenza fusion peptide mostly adopts helical structures with a pronounced kink at residues 11−13 with both N-terminal and C-terminal helices oriented mostly parallel to the membrane surface. A charged C-terminus and the presence of a charge C-terminal tag significantly alters the conformational sampling of the fusion peptide and results in more diverse conformational ensembles that include obliquely inserted N-terminal peptide structures. Protonation of acidic residues also affects the conformational sampling, however, based on pKa shift estimates the overall effect of pH = 5 on the conformational sampling of the influenza fusion peptide appears to be only minor.

1 aPanahi, Afra1 aFeig, Michael uhttps://icer.msu.edu/research/publications/conformational-sampling-influenza-fusion-peptide-membrane-bilayers-function01129nas a2200157 4500008004100000245011900041210006900160260001200229300001000241490000800251520054300259100001800802700002200820700001700842856011200859 2010 eng d00aConformational Sampling of S- and R-Warfarin in Polar Solvents: Implications for Stereoselective Complex Formation0 aConformational Sampling of S and RWarfarin in Polar Solvents Imp c06/2010 a41-510 v9493 aMolecular dynamics simulations of the open side chain conformation of ionic and neutral S- and R-warfarin in aqueous solvent and acetonitrile are described. In all cases, warfarin is found to sample a broad conformational ensemble with major and minor states due to torsional degrees of freedom associated with its side chain. Thermodynamics and kinetics of transitions between these states are analyzed from the simulations and interpreted in the context of stereoselective interactions in biological and non-biological complexes.

1 aFeig, Michael1 aGebreyohannes, K.1 aMcGuffin, V. uhttps://icer.msu.edu/research/publications/conformational-sampling-s-r-warfarin-polar-solvents-implications00548nas a2200157 4500008004100000245006600041210006500107260001200172300001600184490000800200100001300208700002000221700002300241700002000264856010600284 2010 eng d00aDefect-induced rigidity enhancement in layered semiconductors0 aDefectinduced rigidity enhancement in layered semiconductors c07/2010 a1200 - 12030 v1501 aRak, Zs.1 aMahanti, S., D.1 aMandal, Krishna, C1 aFernelius, N.C. uhttps://icer.msu.edu/research/publications/defect-induced-rigidity-enhancement-layered-semiconductors00620nas a2200145 4500008004100000245012900041210006900170260001200239490000700251100001300258700002000271700002300291700002000314856014000334 2010 eng d00aDoping dependence of electronic and mechanical properties of GaSe_{1−x}Te_{x} and Ga_{1−x}In_{x}Se from first principles0 aDoping dependence of electronic and mechanical properties of GaS c10/20100 v821 aRak, Zs.1 aMahanti, S., D.1 aMandal, Krishna, C1 aFernelius, N.C. uhttps://icer.msu.edu/research/publications/doping-dependence-electronic-mechanical-properties-gase-1%E2%88%92xte-x-ga-1%E2%88%92xin-xse01655nas a2200157 4500008004100000245010300041210006900144260001200213300001200225490000700237520107200244100001901316700002401335700001801359856012001377 2010 eng d00aEffect of membrane thickness on conformational sampling of phospholamban from computer simulations0 aEffect of membrane thickness on conformational sampling of phosp c03/2010 a805-8140 v983 aThe conformational sampling of monomeric, membrane-bound phospholamban is described from computer simulations. Phospholamban (PLB) plays a key role as a regulator of sarcoplasmic reticulum calcium ATPase. An implicit membrane model is used in conjunction with replica exchange molecular dynamics simulations to reach μs-ms timescales. The implicit membrane model was also used to study the effect of different membrane thicknesses by scaling the low-dielectric region. The conformational sampling with the membrane model mimicking dipalmitoylphosphatidylcholine bilayers is in good agreement overall with experimental measurements, but consists of a wide variety of different conformations including structures not described previously. The conformational ensemble shifts significantly in the presence of thinner or thicker membranes. This has implications for the structure and dynamics of PLB in physiological membranes and offers what we believe to be a new interpretation of previous experimental measurements of PLB in detergents and microsomal membrane.

1 aSayadi, Maryam1 aTanizaki, Seiichiro1 aFeig, Michael uhttps://icer.msu.edu/research/publications/effect-membrane-thickness-conformational-sampling-phospholamban-computer00640nas a2200205 4500008004100000245005900041210005100100300001200151490000800163653002200171653002300193653002100216100001900237700001700256700001800273700001500291700002100306700001900327856008800346 2010 eng d00aOn the Origin of the Highest Redshift Gamma-Ray Bursts0 aOrigin of the Highest Redshift GammaRay Bursts a117-1260 v70810abinaries: general10agamma rays: bursts10astars: formation1 aBelczynski, K.1 aHolz, D., E.1 aFryer, C., L.1 aBerger, E.1 aHartmann, D., H.1 aO'Shea, B., W. uhttps://icer.msu.edu/research/publications/origin-highest-redshift-gamma-ray-bursts01190nas a2200169 4500008004100000245011200041210006900153260001200222300001400234490000700248520055700255100002500812700002600837700001900863700001800882856012000900 2010 eng d00aPRIMO/PRIMONA: A coarse-grained model for proteins and nucleic acids that preserves near-atomistic accuracy0 aPRIMOPRIMONA A coarsegrained model for proteins and nucleic acid c04/2010 a1266-12810 v783 aThe new coarse graining model PRIMO/PRIMONA for proteins and nucleic acids is proposed. This model combines one to several heavy atoms into coarse-grained sites that are chosen to allow an analytical, high-resolution reconstruction of all-atom models based on molecular bonding geometry constraints. The accuracy of proposed reconstruction method in terms of structure and energetics is tested and compared with other popular reconstruction methods for a variety of protein and nucleic acid test sets. Proteins 2010. © 2009 Wiley-Liss, Inc.

1 aGopal, Srinivasa, M.1 aMukherjee, Shayantani1 aCheng, Yi-Ming1 aFeig, Michael uhttps://icer.msu.edu/research/publications/primo-primona-coarse-grained-model-proteins-nucleic-acids-preserves-near01289nas a2200205 4500008004100000245006500041210006500106260001200171300001100183490000800194520066500202100001800867700001900885700002000904700001600924700001100940700001800951700002000969856009400989 2009 eng d00aConstraints on the Density Dependence of the Symmetry Energy0 aConstraints on the Density Dependence of the Symmetry Energy c03/2009 a1227010 v1023 aCollisions involving {112Sn} and {124Sn} nuclei have been simulated with the improved quantum molecular dynamics transport model. The results of the calculations reproduce isospin diffusion data from two different observables and the ratios of neutron and proton spectra. By comparing these data to calculations performed over a range of symmetry energies at saturation density and different representations of the density dependence of the symmetry energy, constraints on the density dependence of the symmetry energy at subnormal density are obtained. The results from the present work are compared to constraints put forward in other recent analyses.

1 aTsang, M., B.1 aZhang, Yingxun1 aDanielewicz, P.1 aFamiano, M.1 aLi, Z.1 aLynch, W., G.1 aSteiner, A., W. uhttps://icer.msu.edu/research/publications/constraints-density-dependence-symmetry-energy00556nas a2200157 4500008004100000245007300041210006900114260001200183300001400195490000700209100001300216700002000229700002300249700002000272856010600292 2009 eng d00aElectronic structure of substitutional defects and vacancies in GaSe0 aElectronic structure of substitutional defects and vacancies in c02/2009 a344 - 3550 v701 aRak, Zs.1 aMahanti, S., D.1 aMandal, Krishna, C1 aFernelius, N.C. uhttps://icer.msu.edu/research/publications/electronic-structure-substitutional-defects-vacancies-gase00397nas a2200133 4500008004100000245003800041210003800079300001200117490000700129100001900136700001900155700001200174856007700186 2009 eng d00aStrategies for Online Communities0 aStrategies for Online Communities a808-8220 v511 aMiller, K., D.1 aFabian, F., H.1 aLin, S. uhttps://icer.msu.edu/research/publications/strategies-online-communities01629nas a2200157 4500008004100000245004900041210004900090300001100139490000700150520115100157100001201308700002001320700002301340700002001363856008801383 2009 eng d00aTheoretical studies of defect states in GaTe0 aTheoretical studies of defect states in GaTe a0155040 v213 aUsing first principle electronic structure calculations within density functional theory and the supercell model, we have investigated the nature and formation energies of defect states associated with Ga and Te vacancies and Ge and Sn substitutional impurities in GaTe. We have also calculated the band structure of pure GaTe for comparison with systems with defects and also to find out the importance of spin–orbit interaction (SOI) on its band structure. We find that the top valence band at the Γ-point shifts up in energy by 0.1 eV due to the mixing of Te p x –p y and p z bands, this splitting being considerably smaller than in atoms where it is 0.8 eV. From an analysis of charge densities and band structures associated with the defect states, we find that most of them are strongly localized and lie deep in the band gap region. The calculated binding energy of the deep defect state and the ε(−1 / −2) transition level associated with the Ga vacancy appears to be in good agreement with experiment. Formation energy calculations suggest that V Ga is the preferred intrinsic defect in GaTe.

1 aRak, Zs1 aMahanti, S., D.1 aMandal, Krishna, C1 aFernelius, N.C. uhttps://icer.msu.edu/research/publications/theoretical-studies-defect-states-gate-001478nas a2200193 4500008004100000245010300041210006900144260001200213300001400225490000800239520081400247100001901061700002001080700001601100700001101116700001801127700001801145856012101163 2008 eng d00aThe influence of cluster emission and the symmetry energy on neutron-proton spectral double ratios0 ainfluence of cluster emission and the symmetry energy on neutron c02/2008 a145–1480 v6643 aThe emissions of neutrons, protons and bound clusters from central {124Sn} + {124Sn} and {112Sn} + {112Sn} collisions are simulated using the Improved Quantum Molecular Dynamics model for two different density-dependent symmetry-energy functions. The calculated neutron-proton spectral double ratios for these two systems are sensitive to the density dependence of the symmetry energy, consistent with previous work. Cluster emission increases the double ratios in the low energy region relative to values calculated in a coalescence-invariant approach. To circumvent uncertainties in cluster production and secondary decays, it is important to have more accurate measurements of the neutron-proton ratios at higher energies in the center of mass system, where the influence of such effects is reduced.

1 aZhang, Yingxun1 aDanielewicz, P.1 aFamiano, M.1 aLi, Z.1 aLynch, W., G.1 aTsang, M., B. uhttps://icer.msu.edu/research/publications/influence-cluster-emission-symmetry-energy-neutron-proton-spectral-double00488nas a2200157 4500008004100000245004900041210004900090260001200139300001100151490000700162100001200169700002000181700002300201700002000224856008600244 2008 eng d00aTheoretical studies of defect states in GaTe0 aTheoretical studies of defect states in GaTe c01/2009 a0155040 v211 aRak, Zs1 aMahanti, S., D.1 aMandal, Krishna, C1 aFernelius, N.C. uhttps://icer.msu.edu/research/publications/theoretical-studies-defect-states-gate01291nas a2200169 4500008004100000020002600041245012100067210006900188260004600257490000800303520060900311100001900920700002400939700001500963700001700978856012600995 2007 eng d aISBN13: 978084123843500aNew Alternatives for Accurate Electronic Structure Calculations of Potential Energy Surfaces Involving Bond Breaking0 aNew Alternatives for Accurate Electronic Structure Calculations aWashington, DCbAmerican Chemical Sociegy0 v9583 aThe method of moments of coupled-cluster equations (MMCC) is extended to potential energy surfaces involving multiple bond breaking by developing the quasi-variational (QV) and quadratic (Q) variants of the MMCC theory. The QVMMCC and QMMCC methods are related to the extended CC (ECC) theory, in which products involving cluster operators and their deexcitation counterparts mimic the effects of higher-order clusters. The test calculations for N2 show that the QMMCC and ECC methods can provide spectacular improvements in the description of multiple bond breaking by the standard CC approaches.

1 aPiecuch, Piotr1 aPimienta, I., S. O.1 aFan, P.-D.1 aKowalski, K. uhttps://icer.msu.edu/new-alternatives-accurate-electronic-structure-calculations-potential-energy-surfaces-involving-bond01498nas a2200181 4500008004100000245011900041210006900160260001200229300001200241490000800253520084400261100001901105700001501124700001701139700001501156700002401171856012101195 2006 eng d00aAutomated derivation and parallel computer implementation of renormalized and active-space coupled-cluster methods0 aAutomated derivation and parallel computer implementation of ren c08/2005 a79–970 v1063 aOur recent efforts that have led to an automated derivation and computer implementation of the renormalized and active-space coupled-cluster {(CC)} methods with Tensor Contraction Engine {(TCE)} are summarized. The {TCE-generated} renormalized and active-space {CC} computer codes are parallel and applicable to closed- and open-shell references, enabling accurate calculations of potential energy surfaces along bond-breaking coordinates and excited states displaying a significant multi-reference character. The effectiveness of the new codes in describing electronic quasi-degeneracies is illustrated by the renormalized {CC} calculations of the potential energy curve of {HCl} and the active-space {CC} calculations for the low-lying excited states of the Be3 system. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2006

1 aPiecuch, Piotr1 aHirata, So1 aKowalski, K.1 aFan, P.-D.1 aWindus, Theresa, L. uhttps://icer.msu.edu/automated-derivation-and-parallel-computer-implementation-renormalized-and-active-space-coupled01532nas a2200145 4500008004100000245015700041210006900198260001200267300001400279490000700293520091200300100003001212700001801242856012601260 2006 eng d00aBalancing an accurate representation of the molecular surface in generalized born formalisms with integrator stability in molecular dynamics simulations0 aBalancing an accurate representation of the molecular surface in c04/2006 a719–7290 v273 aDifferent integrator time steps in {NVT} and {NVE} simulations of protein and nucleic acid systems are tested with the {GBMV} {(Generalized} Born using Molecular Volume) and {GBSW} {(Generalized} Born with simple {SWitching)} methods. The simulation stability and energy conservation is investigated in relation to the agreement with the Poisson theory. It is found that very close agreement between generalized Born methods and the Poisson theory based on the commonly used sharp molecular surface definition results in energy drift and simulation artifacts in molecular dynamics simulation protocols with standard 2-fs time steps. New parameters are proposed for the {GBMV} method, which maintains very good agreement with the Poisson theory while providing energy conservation and stable simulations at time steps of 1 to 1.5 fs. © 2006 Wiley Periodicals, Inc. J Comput Chem 27: 719-729, 2006

1 aChocholouscaronová, Jana1 aFeig, Michael uhttps://icer.msu.edu/balancing-accurate-representation-molecular-surface-generalized-born-formalisms-integrator-stability02232nas a2200253 4500008004100000245008800041210006900129260001100198300001400209490000800223520146400231100001901695700001601714700001401730700001701744700001501761700001801776700001401794700001701808700001401825700001801839700001501857856010601872 2006 eng d00aExperimental and Theoretical UV Characterizations of Acetyloacetone and its Isomers0 aExperimental and Theoretical UV Characterizations of Acetyloacet c2/2006 a3920-39260 v1103 aCryogenic matrix isolation experiments have allowed the measurement of the UV absorption spectra of the high-energy non-chelated isomers of acetylacetone, these isomers being produced by UV irradiation of the stable chelated form. Their identification has been done by coupling selective UV-induced isomerization, infrared spectroscopy, and harmonic vibrational frequency calculations using density functional theory. The relative energies of the chelated and non-chelated forms of acetylacetone in the S0 state have been obtained using density functional theory and coupled-cluster methods. For each isomer of acetylacetone, we have calculated the UV transition energies and dipole oscillator strengths using the excited-state coupled-cluster methods, including EOMCCSD (equation-of-motion coupled-cluster method with singles and doubles) and CR-EOMCCSD(T) (the completely renormalized EOMCC approach with singles, doubles, and non-iterative triples). For dipole-allowed transition energies, there is a very good agreement between experiment and theory. In particular, the CR-EOMCCSD(T) approach explains the blue shift in the electronic spectrum due to the formation of the non-chelated species after the UV irradiation of the chelated form of acetylacetone. Both experiment and CR-EOMCCSD(T) theory identify two among the seven non-chelated forms to be characterized by red-shifted UV transitions relative to the remaining five non-chelated isomers.

1 aPiecuch, Piotr1 aCoussan, S.1 aFerro, Y.1 aTrivella, A.1 aRoubin, P.1 aWieczorek, R.1 aManca, C.1 aKowalski, K.1 aWloch, M.1 aKucharski, S.1 aMusial, M. uhttps://icer.msu.edu/experimental-and-theoretical-uv-characterizations-acetyloacetone-and-its-isomers02648nas a2200157 4500008004100000245016100041210006900202260001500271300000900286490000800295520182400303653020902127100001502336700001602351856012302367 2006 eng d00aIntriguing Accuracies of the Exponential Wave Function Expansions Exploiting Finite Two-Body Correlation Operators in Calculations for Many-Electron Systems0 aIntriguing Accuracies of the Exponential Wave Function Expansion c08/31/2006 a3-160 v7683 aFollowing the ideas laid down by Nooijen and Nakatsuji, several authors have considered an intriguing possibility of representing the exact many-electron wave functions by the exponential cluster expansions involving two-body correlation operators. In particular, inspired by the symmetric form of the Horn–Weinstein exact energy formula, and exploiting the variational principle and numerical analysis, we have demonstrated that one can obtain nearly exact ground-state wave functions for a few many-electron systems using the exponential cluster expansion involving a finite two-body operator acting on the Hartree–Fock determinant [P. Piecuch et al., Phys. Rev. Lett. 90 (2003) 113001]. After summarizing these earlier findings and making some additional comments on the nature of the exponential cluster expansions involving two-body correlation operators, we examine the following issues: (i) the improvements in the accuracy and convergence toward the full configuration interaction (CI) limit offered by cluster operators containing two-body as well as one-body components, (ii) the improvements in the accuracy resulting from the use of multi-determinantal reference states, and (iii) the potential accuracy of the exponential wave function expansions involving finite one- and two-body cluster operators in excited-state calculations. All calculations are performed for an eight electron model system, which is simple enough to allow for the exact, full CI, and other electronic structure calculations, which has fewer independent parameters in the Hamiltonian than the dimension of the corresponding full CI problem, and which enables one to examine ground and excited states with a varying degree of configurational quasi-degeneracy by simple changes in the corresponding nuclear geometry.

10axact many-electron wave functions; Generalized coupled-cluster methods; Two-body correlation operators; Nooijen's conjecture; Variational calculations; Multi-determinantal reference states; Excited states1 aFan, P.-D.1 aPiecuch, P. uhttps://icer.msu.edu/intriguing-accuracies-exponential-wave-function-expansions-exploiting-finite-two-body-correlation01686nas a2200133 4500008004100000245010300041210006900144300001400213490000800227520116200235100002401397700001801421856011301439 2006 eng d00aMolecular Dynamics Simulations of Large Integral Membrane Proteins with an Implicit Membrane Model0 aMolecular Dynamics Simulations of Large Integral Membrane Protei a548–5560 v1103 aThe heterogeneous dielectric generalized Born {(HDGB)} methodology is an the extension of the {GBMV} model for the simulation of integral membrane proteins with an implicit membrane environment. Three large integral membrane proteins, the bacteriorhodopsin monomer and trimer and the {BtuCD} protein, were simulated with the {HDGB} model in order to evaluate how well thermodynamic and dynamic properties are reproduced. Effects of the truncation of electrostatic interactions were examined. For all proteins, the {HDGB} model was able to generate stable trajectories that remained close to the starting experimental structures, in excellent agreement with explicit membrane simulations. Dynamic properties evaluated through a comparison of B-factors are also in good agreement with experiment and explicit membrane simulations. However, overall flexibility was slightly underestimated with the {HDGB} model unless a very large electrostatic cutoff is employed. Results with the {HDGB} model are further compared with equivalent simulations in implicit aqueous solvent, demonstrating that the membrane environment leads to more realistic simulations.

1 aTanizaki, Seiichiro1 aFeig, Michael uhttps://icer.msu.edu/molecular-dynamics-simulations-large-integral-membrane-proteins-implicit-membrane-model02773nas a2200145 4500008004100000245019000041210006900231260001200300300000900312490000700321520214700328100001502475700001902490856011802509 2006 eng d00aThe Usefulness of Exponential Wave Function Expansions Employing One- and Two-Body Cluster Operators in Electronic Structure Theory: The Extended and Generalized Coupled-Cluster Methods0 aUsefulness of Exponential Wave Function Expansions Employing One c12/2006 a1-570 v513 aIn this paper, the applicability of exponential cluster expansions involving one- and two-body operators in high accuracy ab initio electronic structure calculations is examined. First, the extended coupled-cluster method with singles and doubles (ECCSD) is tested in the demanding studies of systems with strong quasi-degeneracies, including potential energy surfaces involving multiple bond breaking. The numerical results show that the single-reference ECCSD method is capable of providing a qualitatively correct description of quasi-degenerate electronic states and potential energy surfaces involving bond breaking, eliminating, in particular, the failures and the unphysical behavior of standard coupled-cluster methods in similar cases. It is also demonstrated that one can obtain entire potential energy surfaces with millihartree accuracies by combining the ECCSD theory with the non-iterative a posteriori corrections obtained by using the generalized variant of the method of moments of coupled-cluster equations. This is one of the first instances where the relatively simple single-reference formalism, employing only one- and two-body clusters in the design of the relevant energy expressions, provides a highly accurate description of the dynamic and significant non-dynamic correlation effects characterizing quasi-degenerate and multiply bonded systems. Second, an evidence is presented that one may be able to represent the virtually exact ground- and excited-state wave functions of many-electron systems by exponential cluster expansions employing general two-body or one- and two-body operators. Calculations for small many-electron model systems indicate the existence of finite two-body parameters that produce the numerically exact wave functions for ground and excited states. This finding may have a significant impact on future quantum calculations for many-electron systems, since normally one needs triply excited, quadruply excited, and other higher-than-doubly excited Slater determinants, in addition to all singly and doubly excited determinants, to obtain the exact or virtually exact wave functions.

1 aFan, P.-D.1 aPiecuch, Piotr uhttps://icer.msu.edu/usefulness-exponential-wave-function-expansions-employing-one-and-two-body-cluster-operators01534nas a2200265 4500008004100000245007500041210006900116260001500185300001200200490000800212520073300220100001900953700002000972700001600992700001301008700001701021700001401038700002201052700001801074700001401092700001701106700001301123700001701136856011501153 2005 eng d00aComparison of Low-Order Multireference Many-Body Perturbation Theories0 aComparison of LowOrder Multireference ManyBody Perturbation Theo c04/01/2005 a9 pages0 v1223 aTests have been made to benchmark and assess the relative accuracies of low-order multireference perturbation theories as compared to coupled cluster (CC) and full configuration interaction (FCI) methods. Test calculations include the ground and some excited states of the Be, H(2), BeH(2), CH(2), and SiH(2) systems. Comparisons with FCI and CC calculations show that in most cases the effective valence shell Hamiltonian (H(v)) method is more accurate than other low-order multireference perturbation theories, although none of the perturbative methods is as accurate as the CC approximations. We also briefly discuss some of the basic differences among the multireference perturbation theories considered in this work.

1 aPiecuch, Piotr1 aChaudhuri, R.K.1 aFreed, K.F.1 aHose, G.1 aKowalski, K.1 aWloch, M.1 aChattopadhyay, S.1 aMukherjee, D.1 aRolik, R.1 aSzabados, A.1 aToth, G.1 aSurjan, P.R. uhttps://icer.msu.edu/research/publications/comparison-low-order-multireference-many-body-perturbation-theories01726nas a2200157 4500008004100000245013900041210006900180260001200249300001400261490000800275520111600283100001901399700001701418700001701435856011601452 2005 eng d00aNoniterative Corrections to Extended Coupled-Cluster Energies Employing the Generalized Method of Moments of Coupled-Cluster Equations0 aNoniterative Corrections to Extended CoupledCluster Energies Emp c08/2005 a2191-22130 v1033 aIt is shown that the extended coupled-cluster method with singles and doubles (ECCSD) does not suffer from the non-variational collapse observed in the standard CCSD calculations when multiple bond breaking is examined. This interesting feature of the single-reference ECCSD theory is used to design the non-iterative CC methods with singles, doubles and non-iterative triples and quadruples, which provide a highly accurate and variational description of potential energy surfaces involving multiple bond breaking with computational steps that scale as with the system size. This is accomplished with the help of the generalized version of the method of moments of coupled-cluster equations (GMMCC), which can be used to correct the results of non-standard CC calculations, such as ECCSD. The theoretical considerations are illustrated by the preliminary results of the ECCSD-based GMMCC calculations for triple bond breaking in N2. Keywords: Coupled-cluster theory; Extended coupled-cluster method; Method of moments of coupled-cluster equations; Non-iterative coupled-cluster approaches; Bond breaking

1 aPiecuch, Piotr1 aFan, {P.-D.}1 aKowalski, K. uhttps://icer.msu.edu/research/publications/noniterative-corrections-extended-coupled-cluster-energies-employing02386nas a2200241 4500008004100000245015000041210006900191260001200260300001400272490000800286520138300294653017701677100001901854700001701873700002401890700001501914700002101929700002301950700002201973700001301995700001502008856012102023 2004 eng d00aMethod of moments of coupled-cluster equations: a new formalism for designing accurate electronic structure methods for ground and excited states0 aMethod of moments of coupledcluster equations a new formalism fo c07/2004 a349–3930 v1123 aThe method of moments of coupled-cluster equations {(MMCC),} which provides a systematic way of improving the results of the standard coupled-cluster {(CC)} and equation-of-motion {CC} {(EOMCC)} calculations for the ground- and excited-state energies of atomic and molecular systems, is described. The {MMCC} theory and its generalized {MMCC} {(GMMCC)} extension that enables one to use the cluster operators resulting from the standard as well as nonstandard {CC} calculations, including those obtained with the extended {CC} {(ECC)} approaches, are based on rigorous mathematical relationships that define the many-body structure of the differences between the full configuration interaction {(CI)} and {CC} or {EOMCC} energies. These relationships can be used to design the noniterative corrections to the {CC/EOMCC} energies that work for chemical bond breaking and potential energy surfaces of excited electronic states, including excited states dominated by double excitations, where the standard single-reference {CC/EOMCC} methods fail. Several {MMCC} and {GMMCC} approximations are discussed, including the renormalized and completely renormalized {CC/EOMCC} methods for closed- and open-shell states, the quadratic {MMCC} approaches, the {CI-corrected} {MMCC} methods, and the {GMMCC} approaches for multiple bond breaking based on the {ECC} cluster amplitudes.

10aCoupled-cluster theory - Method of moments of coupled-cluster equations - Renormalized coupled-cluster methods - extended coupled cluster theory - Potential energy surfaces1 aPiecuch, Piotr1 aKowalski, K.1 aPimienta, I., S. O.1 aFan, P.-D.1 aLodriguito, M.D.1 aMcGuire}, M., J. {1 aKucharski, S., A.1 aKuś, T.1 aMusial, M. uhttps://icer.msu.edu/research/publications/method-moments-coupled-cluster-equations-new-formalism-designing-accurate