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  • 1
    Electronic Resource
    Electronic Resource
    Solvent effect on vertical electronic transitions by the polarizable continuum model (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 2427-2435 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A recent extension of the polarizable continuum solvation model (PCM) to excited electronic states is applied to the study of solvent effects on electronic transitions, accounting for both electrostatic and nonelectrostatic solute–solvent interactions. A general formalism for nonequilibrium electrostatic solvation is developed, applicable to all PCM versions, and a recent procedure for the quantum-mechanical computation of dispersion and repulsion solute–solvent interactions is implemented and used for the first time in this context. The procedure is applied to the study of the n→π* transition of acetone in aqueous and nonaqueous solvents: nonequilibrium effects are very important in polar environments; also, the inclusion of dispersion and repulsion is mandatory to obtain the correct trend of the solvatochromic shifts. The effect of adding some explicit solvent molecules is also analyzed. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.480808
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  • 2
    Electronic Resource
    Electronic Resource
    Density Functional Calculations of Isotropic Hyperfine Coupling Constants in .beta.-Ketoenolyl Radicals (1994)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 98 (1994), S. 8648-8652 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100086a011
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  • 3
    Electronic Resource
    Electronic Resource
    A new definition of cavities for the computation of solvation free energies by the polarizable continuum model (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 3210-3221 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A set of rules for determining the atomic radii of spheres used to build the molecular cavities in continuum solvation models are presented. The procedure is applied to compute the hydration free energy for molecules containing H, C, N, O, F, P, S, Cl, Br, and I at a computational level (Hartree–Fock with a medium size basis set) allowing the study of relatively large systems. The optimized radii reduce the mean error with respect to the experimental solvation energies below 0.20 kcal/mol for a set of 43 neutral solutes and around 1 kcal/mol for 27 ions. Moreover the correct trends are observed for the solvation energies of homolog series, like the series ammonia–trimethylamine, that are not correctly reproduced by usual solvation models. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.474671
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  • 4
    Electronic Resource
    Electronic Resource
    Exchange functionals with improved long-range behavior and adiabatic connection methods without adjustable parameters: The mPW and mPW1PW models (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 664-675 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Starting from an analysis of the low-density and large gradient regions which dominate van der Waals interactions, we propose a modification of the exchange functional introduced by Perdew and Wang, which significantly enlarges its field of applications. This is obtained without increasing the number of adjustable parameters and retaining all the asymptotic and scaling properties of the original model. Coupling the new exchange functional to the correlation functional also proposed by Perdew and Wang leads to the mPWPW model, which represents the most accurate generalized gradient approximation available until now. We next introduce an adiabatic connection method in which the ratio between exact and density functional exchange is determined a priori from purely theoretical considerations and no further parameters are present. The resulting mPW1PW model allows to obtain remarkable results both for covalent and noncovalent interactions in a quite satisfactory theoretical framework encompassing the free electron gas limit and most of the known scaling conditions. The new functionals have been coded with their derivatives in the Gaussian series of programs, thus allowing fully self-consistent computations of energy and properties together with analytical evaluation of first and second geometry derivatives. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475428
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  • 5
    Electronic Resource
    Electronic Resource
    Diels–Alder reactions: An assessment of quantum chemical procedures (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 8727-8732 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Kinetic and thermodynamic parameters for the Diels–Alder reactions of butadiene with ethylene, formaldehyde, and thioformaldehyde have been computed at a variety of levels of theory with the aim of identifying the most suitable approach for general application. Especially in the presence of heteroatoms, inclusion of electron correlation significantly modifies the geometries of stationary points obtained at the Hartree–Fock level. However, energetic quantities are not very sensitive to the employed geometries provided that the structures of all the species are optimized at the same computational level. On the other hand, both extended basis sets and inclusion of most of the dynamical correlation are needed to obtain reliable energetic quantities. In particular, converged results are obtained only using a triple zeta contraction for the valence space and double sets of polarization functions on nonhydrogen atoms, whereas f functions and hydrogen polarization functions play a minor role. Among cheaper approaches hybrid density functional/Hartree–Fock methods coupled with polarized split valence basis sets provide remarkable geometries and, especially, energetic quantities. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473933
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  • 6
    Electronic Resource
    Electronic Resource
    A direct procedure for the evaluation of solvent effects in MC-SCF calculations (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 5295-5302 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We extended the polarizable continuum solvation model to multi-configurational complete active space as implemented in the Gaussian program. The present formulation allows energy and gradient computations in a unified approach for closed and open shell systems: in this work we present the formal derivation and check the results with energy calculations and geometry optimizations in solution, for ground and excited electronic states. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479788
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  • 7
    Electronic Resource
    Electronic Resource
    Analytical second derivatives of the free energy in solution by polarizable continuum models (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 6246-6254 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present an algorithm for the analytical evaluation of free energy second derivatives with respect to nuclear displacements in the framework of the polarizable continuum model (PCM). The algorithm has been obtained for two recently proposed PCM versions, namely the conductor-like and the isotropic integral equation formalism models, and it can be applied to Hartree–Fock and to density functional SCF calculations on closed- and open-shell systems. The formal derivation is shown in detail and a number of numerical tests are presented: the applications show that analytical results are in excellent agreement with those obtained numerically and, as in vacuo, the analytical procedure is far more efficient. The presented algorithm can be routinely used with an upcoming release of the Gaussian program. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.477265
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  • 8
    Electronic Resource
    Electronic Resource
    Conformational behavior of gaseous glycine by a density functional approach (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 364-370 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Density functional calculations with large basis sets have been performed for nine conformers of neutral glycine. The results obtained by standard, even gradient corrected functionals are significantly different from the best post-Hartree–Fock computations and the available experimental data. Inclusion of some Hartree–Fock exchange significantly improves matters, thus providing a promising protocol for the study of noncovalent interactions in biomolecules. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469411
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  • 9
    Electronic Resource
    Electronic Resource
    Characterization of the potential energy surface of the HO2 molecular system by a density functional approach (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 10666-10676 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The potential energy surface for the ground electronic state of the HO2 system has been characterized using extended basis sets with a recently introduced density functional incorporating gradient corrections and some Hartree–Fock exchange. All the structural, thermodynamic and spectroscopic properties of the hydroperoxide radical and of its molecular fragments (OH, O2) are in close agreement with experiment. The saddle points for HO2 isomerization and OO–H dissociation, together with the hydrogen bonded OH–O structure, have been fully characterized. Refined post Hartree–Fock computations have been performed to further validate density functional results. The two series of quantum mechanical computations are in good agreement and suggest some refinement of the most recent semiempirical surfaces developed for dynamical studies. This task can be made easier by the force fields of all the stationary points computed in the present work. These findings together with the very favorable scaling of the computations with the number of electrons suggest that the density functional approach is a promising theoretical tool for the study of reactions involving large, chemically significant species. © 1994 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467880
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  • 10
    Electronic Resource
    Electronic Resource
    Proton transfer in the ground and lowest excited states of malonaldehyde: A comparative density functional and post-Hartree–Fock study (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 105 (1996), S. 11007-11019 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Intramolecular proton transfer in the ground and the lowest two excited electronic states of malonaldehyde has been investigated by using density functional and post-Hartree–Fock methods. Our best estimates of the energy barriers governing proton transfer in the ground and lowest triplet state are quite low (4.3 and 6.6 kcal/mol, respectively), whereas a significantly higher barrier (12.0 kcal/mol) is obtained for the second triplet state. The coupled cluster approach provides reliable results already with relatively compact basis sets, its only drawback being the very unfavorable scaling with the number of active electrons. Among the cheaper methods, those based on the many-body perturbative approach provide good results for the ground electronic state, but their performances strongly deteriorate for excited states. The overestimation of correlation energy by conventional density functional methods produces an excessive degree of conjugation in the backbone of malonaldehyde with the consequent underestimation of energy barriers governing proton transfer. A more coherent picture is offered by a hybrid density functional/Hartree–Fock approach, which couples good structural predictions with a reduced, although still not negligible, underestimation of energy barriers. Furthermore, different electronic states are described with comparable accuracy. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.472900
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