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  • 1
    Electronic Resource
    Electronic Resource
    Fluxionality and low-lying transition structures of the water trimer (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 5228-5238 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The minimum energy structure of the cyclic water trimer, its stationary points, and rearrangement processes at energies 〈1 kcal/mol above the global minimum are examined by ab initio molecular orbital theory. Structures corresponding to stationary points are fully optimized at the Hartree–Fock and second-order Møller–Plesset levels, using the 6-311++G(d,p) basis; each stationary point is characterized by harmonic vibrational analyses. The lowest energy conformation has two free O–H bonds on one and the third O–H bond on the other side of an approximately equilateral hydrogen-bonded O...O...O (O3) triangle. The lowest energy rearrangement pathway corresponds to the flipping of one of the two free O–H bonds which are on the same side of the plane across this plane via a transition structure with this O–H bond almost within the O3 plane. Six distinguishable, but isometric transition structures of this type connect six isometric minimum energy structures along a cyclic vibrational-tunneling path; neighboring minima correspond to enantiomers. The potential energy along this path has C6 symmetry and a very low barrier V6=0.1±0.1 kcal/mol. This implies nearly free pseudorotational interconversion of the six equilibrium structures. The corresponding anharmonic level structure was modeled using an internal rotation Hamiltonian. Two further low-energy saddle points on the surface are of second and third order; they correspond to crown-type and planar geometries with C3 and C3h symmetries, respectively. Interconversion tunneling vibrations via these stationary points are also important for the water trimer dynamics. A unified and symmetry-adapted description of the intermolecular potential energy surface is given in terms of the three flipping coordinates of the O–H bonds. Implications of these results for the interpretation of spectroscopic data are discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.465991
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  • 2
    Electronic Resource
    Electronic Resource
    Intermolecular bonding and vibrations of phenol⋅H2O (D2O) (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 3763-3776 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Extensive ab initio calculations of the phenol⋅H2O complex were performed at the Hartree–Fock level, using the 6-31G(d,p) and 6-311++G(d,p) basis sets. Fully energy-minimized geometries were obtained for (a) the equilibrium structure, which has a translinear H bond and the H2O plane orthogonal to the phenol plane, similar to (H2O)2; (b) the lowest-energy transition state structure, which is nonplanar (C1 symmetry) and has the H2O moiety rotated by ±90°. The calculated MP2/6-311G++(d,p) binding energy including basis set superposition error corrections is 6.08 kcal/mol; the barrier for internal rotation around the H bond is only 0.4 kcal/mol. Intra- and intermolecular harmonic vibrational frequencies were calculated for a number of different isotopomers of phenol⋅H2O. Anharmonic intermolecular vibrational frequencies were computed for several intermolecular vibrations; anharmonic corrections are very large for the β2 intermolecular wag. Furthermore, the H2O torsion τ around the H-bond axis, and the β2 mode are strongly anharmonically coupled, and a two-dimensional τ/β2 potential energy surface was explored. The role of tunneling splitting due to the torsional mode is discussed and tunnel splittings are estimated for the calculated range of barriers. The theoretical studies were complemented by a detailed spectroscopic study of h-phenol⋅H2O and d-phenol⋅D2O employing two-color resonance-two-photon ionization and dispersed fluorescence emission techniques, which extends earlier spectroscopic studies of this system. The β1 and β2 wags of both isotopomers in the S0 and S1 electronic states are newly assigned, as well as several other weaker transitions. Tunneling splittings due to the torsional mode may be important in the S0 state in conjunction with the excitation of the intermolecular σ and β2 modes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.464055
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  • 3
    Electronic Resource
    Electronic Resource
    Intermolecular bonding and vibrations of 2-naphthol⋅H2O (D2O) (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 1469-1481 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A combined experimental and theoretical study of the 2-naphthol⋅H2O/D2O system was performed. Two different rotamers of 2-naphthol (2-hydroxynaphthalene, 2HN) exist with the O–H bond in cis- and trans-position relative to the naphthalene frame. Using Hartree–Fock (HF) calculations with the 6-31G(d,p) basis set, fully energy-minimized geometries were computed for both cis- and trans-2HN⋅H2O of (a) the equilibrium structures with trans-linear H-bond arrangement and Cs symmetry and (b) the lowest-energy transition states for H atom exchange on the H2O subunit, which have a nonplanar C1 symmetry. Both equilibrium and transition state structures are similar to the corresponding phenol⋅H2O geometries. The H-bond stabilization energies with zero point energy corrections included are ≈5.7 kcal/mol for both rotamers, ≈2.3 kcal/mol stronger than for the water dimer, and correspond closely to the binding energy calculated for phenol⋅H2O at the same level of theory. Extension of the aromatic π-system therefore hardly affects the H-bonding conditions. The barrier height to internal rotation around the H-bond only amounts to 0.5 kcal/mol. Harmonic vibrational analysis was carried out at these stationary points on the HF/6-31G(d,p) potential energy surface with focus on the six intermolecular modes. The potential energy distributions and M-matrices reflect considerable mode scrambling for the deuterated isotopomers. For the a' intermolecular modes anharmonic corrections to the harmonic frequencies were evaluated. The β2 wag mode shows the largest anharmonic contributions. For the torsional mode τ (H2O H-atom exchange coordinate) the vibrational level structure in an appropriate periodic potential was calculated. On the experimental side resonant-two-photon ionization and dispersed fluorescence emission spectra of 2HN⋅H2O and d-2HN⋅D2O were measured. A detailed assignment of the bands in the intermolecular frequency range is given, based on the calculations. The predicted and measured vibrational frequencies are compared and differences discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.465316
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  • 4
    Electronic Resource
    Electronic Resource
    The molecular and electronic structure of s-tetrazine in the ground and first excited state: A theoretical investigation (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 7048-7057 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The ground- and first excited state of s-tetrazine arising from a π*←n excitation (1Ag,1B3u) have been studied using the complete active space (CASSCF) and the second order multiconfiguration perturbation theory (CASPT2) ab initio methods. The focus of this study is on the effect of the electronic excitation on the molecular structure and on those electronic properties which are important to model the solvatochromatic behavior of the molecule in polymer matrices as used in permanent hole burning experiments. Since the accurate computation of excited state molecular properties represents a major challenge for today's numerical quantum chemistry, some technical aspects are also considered. The present study shows that the change in geometry upon electronic excitation is small. This is in partial contradiction with the experimental studies which however disagree among themselves [see K. K. Innes, I. G. Ross, and W. R. Moomaw, J. Mol. Spectrosc. 132, 492 (1988), and R. E. Smalley, L. Wharton, and D. H. Levi, ibid. 66, 375 (1977)]. This study also confirms that the first excited state equilibrium structure is of D2h symmetry. In an earlier theoretical study it was found that the D2h symmetry structure may represent a saddle point rather than a minimum on the excited state potential surface [see A. C. Scheiner and H. F. Schaefer III, J. Chem. Phys. 87, 3539 (1987)]. In the first excited state, we observe an increase of the mean polarizability of s-tetrazine along with an enhanced anisotropy. The change in the polarizability is almost exclusively in the "in-plane'' components of the tensor; the polarizability in the vertical direction is nearly unchanged. This observation questions recent experimental results reported for this molecule [see S. Heitz, D. Weidnauer, and A. Hese, J. Chem. Phys. 95, 7952 (1991)]. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470332
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  • 5
    Electronic Resource
    Electronic Resource
    Low-lying stationary points and torsional interconversions of cyclic (H2O)4: An ab initio study (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 6114-6126 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The global and local minima, stationary points, and torsional rearrangement processes of cyclic homodromic (H2O)4 were studied on its four-dimensional torsional intermolecular potential energy surface. Eight different energetically low-lying torsional stationary point structures were found by ab initio theory, and fully structure-optimized at the second-order Møller–Plesset level, using large basis sets. Second-order energies close to the one-particle basis set limit were obtained at these geometries using the explicitly correlated Møller–Plesset method. The effects of higher-order correlation energy terms were investigated by coupled cluster theory, and terms beyond second order were found to cancel in good approximation. The S4 symmetric global minimum has a square and almost planar O...O...O...O arrangement with free O–H bonds alternating "up'' and "down'' relative to this plane, with two isometric versions of this structure. Another torsional conformer with two neighboring up O–H bonds followed by two neighboring down O–H bonds is a local minimum, 0.93 kcal/mol above the global minimum. The four versions of this structure are connected to the global minima via two distinct but almost degenerate first-order torsional saddle points, which occur as two sets of eight isometric versions each, both about 1.24 kcal/mol above the global minimum. Yet another set of eight second-order saddle points lies at 1.38 kcal/mol. The structure with three O–H bonds up and one down is not a stationary point, while the structure with all four O–H bonds on the same side of the plane is a first-order saddle point.The fully planar C4h symmetric structure is a fourth-order stationary point 2.8 kcal/mol above the minimum. The torsional interconversion paths between this multitude of points are complex, and are discussed in three-dimensional spaces of symmetry-adapted torsional coordinates, and also in a network representation. The torsional normal-mode eigenvectors point fairly directly along the torsional interconversion pathways, but the harmonic frequencies are well below the corresponding barriers. Tunneling interconversion between torsional conformers is, hence, less important than for the water trimer. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470439
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  • 6
    Electronic Resource
    Electronic Resource
    An ab initio derived torsional potential energy surface for (H2O)3. II. Benchmark studies and interaction energies (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 1085-1098 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A torsional potential energy surface for the cyclic water trimer was calculated at the level of second-order Møller–Plesset perturbation theory. For the construction of this ab initio surface, the first-order wave function was expanded in a many-electron basis which linearly depends on the interelectronic coordinates r12. The one-electron basis of Gaussian orbitals was calibrated on the water monomer and dimer to ensure that the ab initio surface computed represents the (near- ) basis set limit for the level of theory applied. The positions of the free O—H bonds are described by three torsional angles. The respective three-dimensional torsional space was investigated by 70 counterpoise corrected single-point calculations for various values of these angles, providing a grid to fit an analytical representation of the potential energy surface. The four symmetry unique stationary points previously found at the Hartree–Fock and conventional Møller–Plesset levels [Schütz et al., J. Chem. Phys. 99, 5228 (1993)] were studied in detail: Relative energies of the structures were calculated by applying second-order Møller–Plesset and coupled cluster methods; harmonic vibrational frequencies were calculated at the second-order Møller–Plesset level with a 6-311++G(d,p) basis set at these stationary points. It is expected that the present torsional potential energy surface for the water trimer will play an important role in the understanding of the vibrational transitions observed by far-infrared vibration–rotation–tunneling spectroscopy in terms of a nearly free pseudorotational interconversion on a cyclic vibrational–tunneling path. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470701
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  • 7
    Electronic Resource
    Electronic Resource
    Intermolecular vibrations of phenol⋅(H2O)3 and d1-phenol⋅(D2O)3 in the S0 and S1 states (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 6350-6361 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report a combined spectroscopic and theoretical investigation of the intermolecular vibrations of supersonic jet-cooled phenol⋅(H2O)3 and d1-phenol⋅(D2O)3 in the S0 and S1 electronic states. Two-color resonant two-photon ionization combined with time-of-flight mass spectrometry and dispersed fluorescence emission spectroscopy provided mass-selective vibronic spectra of both isotopomers in both electronic states. In the S0 state, eleven low-frequency intermolecular modes were observed for phenol⋅(H2O)3, and seven for the D isotopomer. For the S1 state, several intermolecular vibrational excitations were observed in addition to those previously reported. Ab initio calculations of the cyclic homodromic isomer of phenol⋅(H2O)3 were performed at the Hartree–Fock level. Calculations for the eight possible conformers differing in the position of the "free'' O–H bonds with respect to the almost planar H-bonded ring predict that the "up–down–up–down'' conformer is differentially most stable. The calculated structure, rotational constants, normal-mode eigenvectors, and harmonic frequencies are reported. Combination of theory and experiment allowed an analysis and interpretation of the experimental S0 state vibrational frequencies and isotope shifts. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470416
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  • 8
    Electronic Resource
    Electronic Resource
    Erratum: Fluxionality and low-lying transition structures of the water trimer [J. Chem. Phys. 99, 5228 (1993)] (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 1780-1780 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.467292
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  • 9
    Electronic Resource
    Electronic Resource
    Low-order scaling local electron correlation methods. III. Linear scaling local perturbative triples correction (T) (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 9986-10001 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A new method for the perturbative calculation of the correlation energy due to connected triple excitations (T) in the framework of local coupled cluster theory is presented, for which all computational resources scale linearly with molecular size. One notable complication in the formalism for connected triples introduced by the local approach is the nondiagonality of the Fock matrix in the localized MO (LMO) and projected AO (PAO) basis, which leads to couplings between individual triples amplitudes via the internal–internal and external–external blocks of the Fock matrix, respectively. Further complications and couplings arise due to the nonorthogonality of the PAOs. While the couplings via the external–external block can easily be dealt with, this is more difficult for the internal–internal couplings. In a previous paper we already published preliminary results of an approximation of the method, which neglects these internal–internal couplings entirely and recovers about 97% of the total local triples correlation energy. In the present work we implemented the "full" local (T) method, which involves the iterative solution of a system of linear equations for the triples amplitudes to take the internal–internal couplings fully into account. Moreover, a further variant of the method was implemented, which approximates the internal–internal couplings at the level of first-order perturbation theory with respect to the off-diagonal elements of the Fock matrix in LMO basis, thus avoiding the need for an iterative solution of the triples equations and storage of the triples amplitudes. The latter variant reliably recovers more than 99% of the full local triples energy. Test calculations with more than 1000 basis functions and over 300 correlated electrons are presented, showing a speedup of about 106 relative to the estimated time of a corresponding conventional (T) calculation. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1323265
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  • 10
    Electronic Resource
    Electronic Resource
    Low-order scaling local correlation methods II: Splitting the Coulomb operator in linear scaling local second-order Møller–Plesset perturbation theory (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 9443-9455 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A novel multipole approximation for the linear scaling local second-order Møller–Plesset perturbation theory (MP2) method is presented, which is based on a splitting of the Coulomb operator into two terms. The first one contains the singularity and is rapidly decaying with increasing distance. It is treated by a conventional two-electron transformation, where the rapid decay leads to significant savings. The second term is long range, but nonsingular and can therefore be approximated by a multipole expansion. Reliability, accuracy, and efficiency of this method are demonstrated by an extensive benchmark study. It is shown that the goal to further improve the efficiency of the existing linear scaling local MP2 algorithm has been achieved. Moreover, the new method is a promising starting point for future developments, such as coupling of MP2 with density functional theory. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1321295
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