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  • 1
    Electronic Resource
    Electronic Resource
    Cooperative (nonpairwise) effects in water trimers: An ab initio molecular orbital study (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 6628-6638 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: High levels of ab initio molecular orbital theory were used to study the structures and binding energies of water trimers. These calculations included HF/6-311++G(2df,2p) geometry optimizations for the 17 structures considered. Harmonic vibrational energies were obtained at the HF/6–311++G(2d,2p) level. The HF potential energy surface present three minima whose geometries were refined at the MP2/6–311+G(d,p) level. The global minimum corresponds to an asymmetric cyclic structure which presents significant cooperative effects with respect to the Cs dimer. To properly describe these nonpairwise effects, ZPE (zero point energy) and correlation corrections must be taken into account. They are reflected in a stiffer intermolecular potential, shorter O–O distances, longer donor O–H bond lengths, larger energies per hydrogen bond (HB), and greater shifts of the donor O–H bond stretching frequencies than the Cs dimer. Contrarily, the other two local minima present HBs which are weaker than those of the dimer. These nonpairwise effects upon trimerization are mirrored in the topological characteristics of the electronic charge distributions of these clusters and there is a good linear relationship between the energies per HB and the charge density at the HB critical point. The binding energies for the three minima were evaluated at the MP4SDQ/6–31+G(2d,2p) level using the MP2 optimized geometries. For the remaining structures considered they were obtained at the MP2/6–311++G(2d,2p) level using the corresponding HF optimized geometries. We have also shown that nonpairwise effects can be qualitatively explained in terms of acid-base arguments.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463666
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  • 2
    Electronic Resource
    Electronic Resource
    Potential energy surfaces of C2v and D3h ozone complexes with Li+ (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 253-265 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Ab initio molecular orbital calculations have been used to study the most important features of the potential energy surfaces corresponding to Li+ association to C2v and D3h ozone. For this purpose highly correlated techniques [CASSCF, QCISD, and QCISD(T)] have been used. Our results confirm that these highly correlated techniques are unavoidable in so far as a correct description of ozone–Li+ complexes is needed. Good agreement between CASSCF and QCI results is attained for C2v ozone when the UHF function is taken as the reference function and triple excitations are considered in the QCI treatment. Results for D3h ozone are in agreement only when a proper description of the π correlation is included in the CASSCF treatment. Interactions with Li+ are stronger for open chain ozone than for the cyclic isomer. Thus on complex formation the energy gap between C2v and D3h ozone increases. There exist three structures which are predicted to lie very close in energy. The global energy minimum corresponds to an isomer in which the Li+ bridges both terminal oxygens, but other structures corresponding to the attachment of Li+ to one of the terminal oxygens and to an insertion of the Li+ in one of the O–O bonds, respectively, are about 1 kcal/mol less stable. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469638
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  • 3
    Electronic Resource
    Electronic Resource
    Structure, vibrational frequencies, and thermodynamic properties of hydrogen peroxide dimers: An ab initio molecular orbital study (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 2871-2877 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: High levels of ab initio molecular orbital theory were used to study the structures, binding energies, vibrational frequencies, and equilibrium constants of hydrogen peroxide dimers. The geometries of the different initial structures considered were optimized at the HF/6–311++G(2d,2p) level of theory. Five different stationary points have been characterized at this level, but only two of them were minima. The geometries of these two minima were refined at the MP2/6–311+G(d,p) level. Their vibrational frequencies, calculated at the same level of theory, show a sizeable redshift of the stretching vibrations of the proton donors. The global minimum corresponds to a six-membered ring of Ci symmetry, while the second minimum is a five-membered ring, which lies about 1.1 kcal mol−1 above the global one. The formation of the latter implies a considerable enhancement of the dipole moment. The binding energies of these two species were obtained at the QCISD(T)/6–311+G(2d,p) level using the MP2 optimized geometries. The equilibrium dimerization constants for hydrogen peroxide are considerably smaller than those for water, due to significant entropic effects. A topological analysis of the electronic charge densities of the dimers shows that both cyclic minima present weaker hydrogen bonds than noncyclic dimers.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466429
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  • 4
    Electronic Resource
    Electronic Resource
    Is the depletion of ozone by HSO an exothermic process? (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 101 (1994), S. 2175-2179 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The Gaussian-2 (G2) theoretical procedure, based on ab initio molecular orbital theory, as well as other high level ab initio calculations [QCISD(T) and CCSD(T)/6–311++G(3df,2pd, and QCISD(T)/6–311+G(5d2f,2p)] which avoid the additivity scheme of this theory, are used to estimate the heat of formation of HSO. For this purpose, we propose also, as a further improvement of the standard G2 scheme, to evaluate the residual correlation effects by using the QCISD(TQ) method, which is exact in fifth order rather than the usual QCISD(T) procedure. The heat of formation of HSO at 0 K estimated using different reactive processes is −4.2±1.3 kcal/mol. The most significant consequence is that the depletion of ozone by HSO is predicted to be slightly endothermic rather than exothermic, as it has been concluded in other theoretical studies. We have also found that HSO is more stable than SOH, but the energy gap between both isomers predicted by our calculations is smaller than previous reported values.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467723
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  • 5
    Electronic Resource
    Electronic Resource
    The performance of density-functional theory in challenging cases: Halogen oxides (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 6131-6140 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Halogen dioxides (FOO, ClOO, BrOO, OClO, OBrO), their cationic and anionic derivatives and two isomers of ClO3 have been studied by means of density-functional theory (DFT) and the results compared with those from high level ab initio molecular orbital calculations. Three different density functionals (SVWN, B3LYP, and G96LYP) combined with a 6-311+G(2df ) basis set were used to obtain geometries and vibrational frequencies, which were then compared with MP2 (second-order Moller–Plesset), QCISD, and CCSD(T) (coupled-cluster single double triple) results. The B3LYP/6-311+G(2df ) calculations generally give geometries and frequencies in excellent agreement with those calculated from high level ab initio calculations such as CCSD(T). Exceptions, such as ClOO and BrOO, arise when high spin contamination at B3LYP level produces spurious results. Atomisation enthalpies evaluated at B3LYP/6-311+G(3df ) level of theory are observed to be in good agreement with the experimental values. In some particular cases this agreement is better than that obtained at CCSD(T)/6-311+G(3df ) level. For ionization enthalpies the CCSD(T) calculations seem to be superior to the DFT ones. Wave function instabilities [with respect to the UHF (unrestricted Hartree–Fock) transformation in the case of the cations and internal symmetry breaking in the case of the OXO (X=Cl, Br) compounds and the C3v isomer of ClO3] are observed less frequently when DFT methods are used. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481214
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  • 6
    Electronic Resource
    Electronic Resource
    Ab initio and density functional theory calculations on the protonated species of As4 clusters (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 8957-8963 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: High level ab initio and density functional theory approaches have been used to study the structure and bonding of the stationary points of the As4H+ potential energy surface. The geometries and the harmonic vibrational frequencies of the different species investigated were obtained through the use of all-electron MP2(full) and B3LYP methods using 6-311G(d) basis expansions. Final energies were obtained in G2(MP2) and B3LYP/6-311+G(3df,2p) calculations. The global minimum is the result of the side protonation of As4 tetrahedral molecule. The corner-protonated species is significantly less (∼23 kcal/mol) stable, while the face-protonated form is a second-order saddle point. The open structure 5 resulting from the fission of two As–As bonds lies 18 kcal/mol above the global minimum. In the global minimum the proton is covalently attached to two As atoms through the formation of a three-center delocalized bonding orbital. The relative stability of this "nonclassical" structure is enhanced with respect to that of the homologous P4H+ system, indicating that the heavier elements prefer divalent arrangements. As a consequence, the As4 cluster is predicted to be about 8 kcal/mol more basic than P4 tetramer in the gas phase. The harmonic vibrational frequencies of the As4H+ side-protonated species is consistent with the existence of the three-center As–H–As covalent bond. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.476341
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  • 7
    Electronic Resource
    Electronic Resource
    Study of the methanol trimer potential energy surface (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 3592-3601 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The potential energy surface of methanol trimer has been studied through the use of high-level ab initio calculations and density functional methods. The geometries have been optimized at the MP2/6-311+G(d,p) and B3LYP/6-311+G(d,p) levels of theory. The harmonic vibrational frequencies were obtained at the latter level. The final energies for the most stable (CH3OH)n (n=1,3) clusters were calculated in the framework of the G2(MP2,SVP) theory. For these and all the other structures the final energies were also obtained using the B3LYP/6-311++G(3df,2p) approach. Three local minima have been located. The global minimum corresponds to a cyclic structure with two methyl groups on one side of the O–O–O plane and the third one on the other side. The bowl conformer, where the three methyl groups are on the same side of the O–O–O plane, is predicted to be only 0.8 kcal/mol less stable than the global minimum. The third local minimum, where one of the monomers behaves as a biacceptor is predicted to lie much higher in energy. Other stationary points associated with a systematic flipping of the methyl groups have been also located. These stationary points, which are transition states or saddle points of higher order, are very close in energy to the global minimum, indicating that the potential energy surface of the methanol trimer is very flat and very similar to that reported before for the water trimer. The calculated enthalpies of association for the dimer and the trimer of methanol are in very good agreement with the experimental values when estimated at the G2(MP2,SVP) level, while they are slightly underestimated when the B3LYP approach is used. The most stable trimer has three different O–H stretching frequencies, in agreement with the experimental evidence. The calculated frequency shifts are only in rough agreement with the experimental values. Cooperative effects are significant in the methanol trimer. They are reflected in larger frequency shifts, greater lengthening of the O–H bonds, shorter O(centered ellipsis)O distances than in the dimer. The G2(MP2,SVP) calculated additive interaction energy is also significantly large. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.474486
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  • 8
    Electronic Resource
    Electronic Resource
    Feshbach and pseudopotential theories. A useful analogy (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 6635-6642 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: An analogy is drawn between the well-known Feshbach theory on one hand, and on the other the reputedly cruder pseudopotential technique for two-electron atomic and molecular systems. We show how both formalisms can be treated on an equal footing by using a generalized Phillips–Kleinman formalism for the second. Among other benefits stemming from this analogy, the simplicity and versatility of pseudopotential methods can be used to considerably simplify the calculation of positions (couplings) and widths involving two-electron autoionizing atomic (molecular) states.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453449
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  • 9
    Electronic Resource
    Electronic Resource
    Nonadiabatic ionic–covalent transitions. Exponential-linear model for the charge exchange and neutralization reactions Na+H (arrow-right-and-left) Na++H− (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 84 (1986), S. 147-151 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A previous study of charge exchange processes taking place through ionic–covalent transitions is extended to the case of Na+H and Na++H− collisions. A five-state molecular expansion, with the inclusion of two-electron translation factors, is employed to calculate the charge exchange and neutralization cross sections. Transitions at the first two pseudocrossings between the energy curves, practically determine the cross sections in the energy range 0.16–5 keV amu−1. We also show that the widely used multichannel Landau–Zener theory is totally inadequate, to treat these transitions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.450190
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