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  • American Institute of Physics (AIP)  (49,884)
  • 2020-2021
  • 1995-1999  (49,884)
  • 1935-1939
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  • 1
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    El Niño Dynamics (1998)
    American Institute of Physics (AIP)
    In:  Physics Today, 51 (12). p. 32.
    Details
    Publication Date: 2021-02-25
    Description: Bringer of storms and droughts, the El Niño∕Southern Oscillation results from the complex, sometimes chaotic interplay of ocean and atmosphere.
    Type: Article , PeerReviewed
    Format: text
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    PAPER (OCEANREP)
  • 2
    Electronic Resource
    Electronic Resource
    Property calculations using perturbed orbitals via state-specific multireference coupled-cluster and perturbation theories (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 3820-3831 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this paper we apply the recently developed state-specific multireference coupled-cluster and perturbation theories to calculate electrical properties such as dipole moment and static polarizability using perturbed orbitals in finite fields. The theories are built on complete active space reference functions, and are designed to treat quasidegeneracy of varying degrees while bypassing the intruder problem. Numerical results are presented for the model systems H4 with trapezoidal geometry and the lowest two singlet states of CH2. Both the systems require a multireference formulation due to quasidegeneracy. In the field-free situation, the former encounters intruders at an intermediate trapezoidal geometry in the traditional treatment using effective Hamiltonians, while the latter shows a pronounced multireference character in the two singlet states. This affects the response properties in the presence of a perturbing field. A comparison with the full CI results in the same basis indicates the efficacy of the state-specific methods in wide ranges of geometries, even when the traditional effective Hamiltonian based methods fail due to intruders. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479685
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  • 3
    Electronic Resource
    Electronic Resource
    Interaction optimized basis sets for correlated ab initio calculations on the water dimer (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 3812-3819 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A compact basis set is constructed for the water dimer by focusing directly on an optimal description of the counterpoise-corrected interaction energy (ΔE) rather than on the total energy of the fragments. The optimization criterion is that the basis set should be of uniform accuracy, i.e., the truncation error in ΔE due to the basis set incompleteness should be the same for all symmetry types at all sites. Aiming at a truncation error of 10 μhartree per symmetry at the SCF+MP2 (self-consistent field+Møller–Plesset second-order) frozen core level the resulting interaction optimized basis set comprises 249 functions. The composition of this IO249 set is O/5s3p4d3f2g1h, H(donor)/2s4p1d, H(else)/2s3p, bond function set/3s3p2d1f. An all-electron variant, IO275, is described as well. A recipe to obtain interaction optimized sets for other systems is given. The set IO249 yields a ΔE(fc) value at the Feller–Frisch geometry of −4.87 kcal/mol. Of the many orbital-based calculations that have been reported for this system only Schütz' 1046-function calculation [J. Chem. Phys. 107, 4597 (1997)] was more accurate. The small size of the interaction optimized sets opens the possibility for high-accuracy SCF+MP2 work on larger systems than have been accessible before. It also brings higher-level correlated treatments within reach. An Appendix summarizes two additivity rules which allow the ΔE for a larger basis set to be estimated to very high accuracy from the results of smaller basis sets. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479684
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  • 4
    Electronic Resource
    Electronic Resource
    Z-vector formalism for the Fock space multireference coupled cluster method: Elimination of the response of the highest valence sector amplitudes (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 3832-3836 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In this paper we present Z-vector formalism for the Fock space multireference coupled cluster framework. We present detailed equations describing the derivative effective Hamiltonian, where the response amplitudes of the highest Fock sector are eliminated. We discuss the conditions and approximations under which the formalism is possible. We also discuss the implications and comparison with the similar formalism in the single reference framework. The computational advantages and feasibility are also commented upon in this paper. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479686
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  • 5
    Electronic Resource
    Electronic Resource
    Theoretical study of the electronic ground state of iron(II) porphine. II (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 3837-3845 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Ten low-lying electronic states of Fe(II) porphine, 5A1g, 5Eg, 5B2g, 3A2g, 3B2g, 3Eg(A), 3Eg(B), 1A1g, 1B2g, and 1Eg states, are studied with multiconfigurational second-order perturbation (CASPT2) calculations with complete active space self-consistent field (CASSCF) reference functions with larger active space and basis sets. The enlargement of active space and basis sets has no influence on the conclusion of a previous multireference Møller–Plesset perturbation (MRMP) study. The present CASPT2 calculation concludes that the 5A1g state is the ground state. A relativistic correction has been performed by the relativistic scheme of eliminating small components (RESC). For energetics, no significant contribution from the relativistic correction was found. The relative energies and orbital energies are not changed appreciably by the introduction of a relativistic correction. The present result does not agree with all the spectroscopic observations, but is consistent with a magnetic moment study. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479687
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  • 6
    Electronic Resource
    Electronic Resource
    High-level ab initio computations of structures and relative energies of two isomers of the CO2 trimer (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 3846-3854 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The two-body interaction energies of the cyclic (C3h) and noncyclic (C2) trimers at the basis set limit (15.55 and 16.55 kJ/mol) were estimated by means of second-order Møller–Plesset perturbation theory including terms linear in the interelectronic distance (MP2-R12 approach). Coupled cluster calculations with single and double substitutions and with noniterative triple excitations [CCSD(T)] were carried out to correct the MP2-R12 two-body interaction energies for the effect of higher excitations, and also for the evaluation of the three-body interaction energies. The calculated bonding energies of the trimers using this model calculation (16.82 and 17.20 kJ/mol) suggest that the two isomers are nearly isoenergetic. The three-body terms are small (only of the order of 1 kJ/mol), stabilizing the cyclic trimer, but destabilizing the noncyclic trimer. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479688
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  • 7
    Electronic Resource
    Electronic Resource
    Density functional theory study on ethanol dimers and cyclic ethanol trimers (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 3855-3861 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The structure and the relative stability of the ethanol dimer and the cyclic ethanol trimer were studied using density functional theory methods. The geometries of the different dimers and trimers were optimized at the B3LYP/6-311+G(d,p) level of theory, while the final energies were obtained at the B3LYP/6-311+G(3df,2p) level. Four different (ethanol)2 complexes were found to be local minima of the potential energy surface, the global minimum being that in which both monomers exhibit a trans conformation. The hydrogen bond (HB) in ethanol dimer is slightly stronger than in methanol dimer, reflecting the enhanced intrinsic basicity of ethanol with regards to methanol. The OH donor stretch appears redshifted by 161 cm−1, while the redshifting undergone by the OH acceptor stretch is negligibly small. The relative stability of the trimers is a function of the number of monomers with a gauche conformation, the global minimum being that in which the three monomers have a trans conformation. As for water and methanol trimers, the three HBs in the cyclic ethanol trimer are not strictly equivalent. Consistently, the redshiftings of the OH stretching frequencies are different. Cooperative effects are sizably large, as reflected in the O(centered ellipsis)O distances, the elongation of the OH donor groups, the charge density at the bond critical points, the frequency shiftings of the OH stretches, and the additivity interaction energy. The most significant features of the vibrational spectra of the monomers, the dimers, and the trimers in the 800–1200 cm−1 region are reasonably well reproduced by our calculations.© 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479689
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  • 8
    Electronic Resource
    Electronic Resource
    Dynamics of the O(3P)+HCl reaction on the 3A″ electronic state: A new ab initio potential energy surface, quasi-classical trajectory study, and comparison to experiment (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 3862-3873 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new potential energy surface for the lowest 3A″ electronic state of the O(3P)+HCl system is presented. This surface is based on electronic energies calculated at the multireference configuration interaction level of theory with the Davidson correction (MR-CI+Q) using the Dunning cc-pVTZ one-electron basis sets. The ab initio energies thus obtained are scaled using the scaled external correlation (SEC) method of Brown and Truhlar. The SEC-scaled energies are fitted to a simple analytical expression to yield a potential energy surface which correlates the reactants O(3P)+HCl(1Σ+) to the products OH(2Π)+Cl(2P). The reaction barrier on this surface lies at an O–H–Cl angle of 131.4° at an energy of 9.78 kcal/mol above the asymptotic O+HCl minimum. This barrier is 1.3 kcal/mol higher than that on the potential energy surface obtained by Koizumi, Schatz, and Gordon (KSG) [J. Chem. Phys. 95, 6421 (1991)] and 1.1 kcal/mol lower than the S2 surface of Ramachandran, Senekowitsch, and Wyatt (RSW) [J. Mol. Struct. (Theochem) 454, 307 (1998)]. The dynamics of the reaction O(3P)+HCl(v=2; j=1,6,9)→OH(v′,j′)+Cl on this potential surface is studied using quasi-classical trajectory (QCT) propagation and the results are compared to the experimental observations of Zhang et al. [R. Zhang, W. J. van der Zande, M. J. Bronikowski, and R. N. Zare, J. Chem. Phys. 94, 2704 (1991)]. The broad distribution of collision energies in the experiment is modeled by computing weighted averages of the quantities of interest with the weighting factor at each collision energy determined by the collision energy distribution. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480264
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  • 9
    Electronic Resource
    Electronic Resource
    Liquid vapor equilibria for an ab initio model for water (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2103-2108 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The vapor–liquid coexistence densities for water near the critical point were determined using a polarizable ab initio based model and grand canonical Monte Carlo simulations combined with the histogram reweighting technique. The predictions of the model used, which is found to give good agreement with experimental data at ambient conditions, are far below the experimental critical temperature and density. The saturation pressure is also overestimated. The source of this discrepancy may be related to the high pressure that the model exhibits even for liquid water. Since there is no fitting to experimental data, it is possible to refine the potential in a systematic way. In particular, an improvement in the sampling of the ab initio calculation for the repulsive part of the intermolecular potential is suggested in order to obtain better agreement with experiment at high temperatures and pressures. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479479
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  • 10
    Electronic Resource
    Electronic Resource
    Modeling of zeolite L crystallization using continuum time Monte Carlo simulations (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 2143-2150 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A model to investigate a gel-to-zeolite transformation as a possible growth mechanism through association and rearrangements of extended precursor structures is developed and solved via continuum time lattice Monte Carlo simulations. The model is used to study the morphology of zeolite L nanocrystals grown from an initial amorphous microporous precursor gel. The model considers gel dissolution to release growth precursor extended structures, precursor migration, precursor–precursor association, precursor incorporation into zeolite nanoparticles, and zeolite dissolution. It is shown that the gel-to-zeolite transformation can occur when dissolved precursor migration is slow compared to precursor incorporation and zeolite dissolution. Under these conditions, the initial gel microstructure (intraparticle porosity) has a significant effect on the zeolite morphology and on the crystallization kinetics. This transformation proceeds in two stages: A zeolitic framework forms initially without long-range order, followed by slow rearrangement of building units into nanocrystalline particles with possible defects. Finally different growth modes are identified as a function of microkinetic parameters and gel morphology. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479484
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