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Henry F Schaefer - One of the best experts on this subject based on the ideXlab platform.

  • the treacherous Potential Energy Hypersurface of agsio
    Journal of Chemical Physics, 2003
    Co-Authors: K Balasubramanian, Henry F Schaefer
    Abstract:

    The AgSiO system has been a source of puzzlement for more than a decade, with experimental and theoretical studies providing bewildering conclusions regarding the structure of the molecule. State-of-the-art coupled cluster and multireference configuration interaction methods have been applied in the present work, in conjunction with techniques designed to incorporate relativistic effects. With the coupled cluster single and double excitation with perturbatively applied triples method [CCSD(T)], the Ag–SiO dissociation Energy is predicted to be 6.8 kcal/mole, with the equilibrium structure being a nearly isosceles triangle. However, a second minimum Ag–O–Si structure with bond angle ∼150° lies at less than 1 kcal/mole. With the multireference CI approach (up to 48 million configurations) the energetic order of these two minima is reversed. In contrast, both second-order perturbation theory and density functional theory predict an Ag–Si–O structure with bond angle ∼115° to be the global minimum. The present...

  • the naphthylcarbene Potential Energy Hypersurface
    Journal of the American Chemical Society, 1997
    Co-Authors: Peter R Schreiner, Paul Von Rague Schleyer And, Henry F Schaefer
    Abstract:

    The naphthylcarbene Potential Energy surface (PES) was examined ab initio, employing self-consistent field (SCF), second-order perturbation theory (MP2), and density functional (Becke3LYP) methods in conjunction with 6-31G*, DZ, DZP, and 6-311+G* basis sets. All stationary structures were characterized by vibrational frequency analyses at the Becke3LYP/6-31G* level; final energies were evaluated at the Becke3LYP/6-311+G*//Becke3LYP/6-31G* + ZPVE level. Cyclobuta[de]naphthalene is the global minimum on this part of the C11H8 PES. Generally, seven-membered benzocarbenes are no minima as they converge to their corresponding allenes. Both 1- and 2-naphthylcarbene have triplet ground states, but the small S−T gaps (ca. 5 kcal mol-1) allow facile rearrangements in the singlet manifold to take place. The triplet rotational barrier for the exo-methylene in 2-naphthylcarbene is relatively small (3.5 kcal mol-1) due to weak π-bonding. At low temperatures, singlet 2-naphthylcarbene equilibrates with 2,3-benzobicyclo...

  • the gaoh hgao Potential Energy Hypersurface and the necessity of correlating the 3d electrons
    Journal of Chemical Physics, 1996
    Co-Authors: Claude A Richards, Yukio Yamaguchi, Henry F Schaefer
    Abstract:

    The ground state Potential Energy Hypersurface of the GaOH–HGaO system has been investigated using high level ab initio molecular electronic structure theory. The geometries and physical properties of two equilibrium structures, one isomerization transition state and one inversion transition state were determined at the self‐consistent field (SCF), configuration interaction with single and double excitations (CISD), coupled cluster with single and double excitations (CCSD), and CCSD with perturbative triple excitations [CCSD(T)] levels of theory with four sets of basis functions. It has been found that freezing the 3d electrons of the Ga atom in the correlation procedures is not appropriate for this system. For the Energy difference ΔE (GaOH–HGaO) the freezing of the 3d electrons results in an error of 25 kcal/mol! The dipole moments, harmonic vibrational frequencies, and infrared (IR) intensities are predicted for the four stationary points. At the highest level of theory employed in this study, CCSD(T) ...

  • the nitrosyl azide Potential Energy Hypersurface a high Energy density boom or bust
    Journal of the American Chemical Society, 1996
    Co-Authors: John Morrison Galbraith, Henry F Schaefer
    Abstract:

    Motivated by the recent isolation and spectroscopic characterization of nitrosyl azide (N4O), we have undertaken an ab initio investigation of the originally reported structure as well as various structural isomers on the Potential Energy Hypersurface. Geometries and harmonic vibrational frequencies have been predicted for the trans-chain isomer along with the 6 π electron Potentially aromatic ring structure with various levels of theory up through the triple-ζ plus double polarization single and double excitation coupled cluster (TZ2P CCSD) method and the multireference configuration interaction method (MRCISD). In addition, estimates are made for extension to higher levels of theory, arriving at final predictions of re(ON1) = 1.176 A, re(N1N2) = 1.472 A, re(N2N3) = 1.272 A, re(N3N4) 1.423, ϑe(N1ON4) = 106.3° for the trans-chain and ring isomers, respectively. Energy relationships, b...

  • the ethylenedione anion elucidation of the intricate Potential Energy Hypersurface
    Journal of Chemical Physics, 1995
    Co-Authors: Russell J Thomas, Henry F Schaefer, Bradley J Deleeuw, Paul Oleary, Brian J Duke, Brian Oleary
    Abstract:

    Ab initio molecular orbital theory has been used to study the controversial Potential Energy surface of the ethylenedione anion C2O−2. Seven different basis sets, the largest being triple zeta plus two polarization functions and one set of higher angular momentum functions (TZ2Pf) in quality, were utilized in conjunction with five correlated methods, the highest‐level being coupled‐cluster theory including single, double, and perturbative triple excitations [CCSD(T)]. Equilibrium geometries and harmonic vibrational frequencies of the predicted 2Au trans‐bent ground state are presented. The Renner–Teller Potential Energy surface resulting from the splitting of the doubly degenerate linear 2Πu transition state into the nondegenerate bent 2Au and linear 2Bu surfaces is also characterized by means of Energy predictions for these three states. Several recent peak assignments in the experimental spectrum, as well as the isotopic shifts associated with them, are supported by theory. A correct description of the ...

Bernhardt L. Trout - One of the best experts on this subject based on the ideXlab platform.

  • Computation of the methane–water Potential Energy Hypersurface via ab initio methods
    Journal of Chemical Physics, 2001
    Co-Authors: Jefferson W. Tester, Bernhardt L. Trout
    Abstract:

    Accurate intermolecular Potentials between hydrocarbons and water are essential for the prediction of properties of systems containing these components. Unfortunately, current experimental techniques are unable to measure directly the interaction Potential between methane and water molecules. Therefore we have used quantum mechanical calculations, both ab initio and density functional theory (DFT) calculations, in order to determine the H2O–CH4 Potential Energy surface (PES) accurately for use in modeling gas hydrates. Ab initio methods were found to be more accurate than DFT methods, which do not account for the substantial dispersion interactions that exist between methane and water. Electron correlation was found to be treated accurately by MP2. However, a large basis set, cc-pVQZ was found to be necessary to compute the binding energies to within 0.1 kcal/mol of the basis set limit. In order to sample accurately the PES, the H2O–CH4 binding Energy was computed at 18 000 points. For these computations ...

  • computation of the methane water Potential Energy Hypersurface via ab initio methods
    Journal of Chemical Physics, 2001
    Co-Authors: Jefferson W. Tester, Bernhardt L. Trout
    Abstract:

    Accurate intermolecular Potentials between hydrocarbons and water are essential for the prediction of properties of systems containing these components. Unfortunately, current experimental techniques are unable to measure directly the interaction Potential between methane and water molecules. Therefore we have used quantum mechanical calculations, both ab initio and density functional theory (DFT) calculations, in order to determine the H2O–CH4 Potential Energy surface (PES) accurately for use in modeling gas hydrates. Ab initio methods were found to be more accurate than DFT methods, which do not account for the substantial dispersion interactions that exist between methane and water. Electron correlation was found to be treated accurately by MP2. However, a large basis set, cc-pVQZ was found to be necessary to compute the binding energies to within 0.1 kcal/mol of the basis set limit. In order to sample accurately the PES, the H2O–CH4 binding Energy was computed at 18 000 points. For these computations ...

Jefferson W. Tester - One of the best experts on this subject based on the ideXlab platform.

  • Computation of the methane–water Potential Energy Hypersurface via ab initio methods
    Journal of Chemical Physics, 2001
    Co-Authors: Jefferson W. Tester, Bernhardt L. Trout
    Abstract:

    Accurate intermolecular Potentials between hydrocarbons and water are essential for the prediction of properties of systems containing these components. Unfortunately, current experimental techniques are unable to measure directly the interaction Potential between methane and water molecules. Therefore we have used quantum mechanical calculations, both ab initio and density functional theory (DFT) calculations, in order to determine the H2O–CH4 Potential Energy surface (PES) accurately for use in modeling gas hydrates. Ab initio methods were found to be more accurate than DFT methods, which do not account for the substantial dispersion interactions that exist between methane and water. Electron correlation was found to be treated accurately by MP2. However, a large basis set, cc-pVQZ was found to be necessary to compute the binding energies to within 0.1 kcal/mol of the basis set limit. In order to sample accurately the PES, the H2O–CH4 binding Energy was computed at 18 000 points. For these computations ...

  • computation of the methane water Potential Energy Hypersurface via ab initio methods
    Journal of Chemical Physics, 2001
    Co-Authors: Jefferson W. Tester, Bernhardt L. Trout
    Abstract:

    Accurate intermolecular Potentials between hydrocarbons and water are essential for the prediction of properties of systems containing these components. Unfortunately, current experimental techniques are unable to measure directly the interaction Potential between methane and water molecules. Therefore we have used quantum mechanical calculations, both ab initio and density functional theory (DFT) calculations, in order to determine the H2O–CH4 Potential Energy surface (PES) accurately for use in modeling gas hydrates. Ab initio methods were found to be more accurate than DFT methods, which do not account for the substantial dispersion interactions that exist between methane and water. Electron correlation was found to be treated accurately by MP2. However, a large basis set, cc-pVQZ was found to be necessary to compute the binding energies to within 0.1 kcal/mol of the basis set limit. In order to sample accurately the PES, the H2O–CH4 binding Energy was computed at 18 000 points. For these computations ...

F Schaefer - One of the best experts on this subject based on the ideXlab platform.

  • naso naos Potential Energy Hypersurface
    Journal of Chemical Physics, 1990
    Co-Authors: F Schaefer
    Abstract:

    For the ground and first excited electronic states of NaOS three minima and three transition states for interconversion have been determined by means of self‐consistent field (SCF) and single and double excitation configuration interation (CISD) analytic gradient methods. The relationships between these structures have been clarified by vibrational analyses of the transition states. One of the NaOS equilibrium geometries is linear and the other two are bent. The chemical bonds between the two component parts of NaOS, Na and SO, for both 2A’ and 2A‘ bent structures have ionic character, although differing in degree. In all studies both double‐zeta‐plus polarization and triple‐zeta‐plus double polarization basis sets were used.

  • NaSO → NaOS Potential Energy Hypersurface
    Journal of Chemical Physics, 1990
    Co-Authors: F Schaefer
    Abstract:

    For the ground and first excited electronic states of NaOS three minima and three transition states for interconversion have been determined by means of self‐consistent field (SCF) and single and double excitation configuration interation (CISD) analytic gradient methods. The relationships between these structures have been clarified by vibrational analyses of the transition states. One of the NaOS equilibrium geometries is linear and the other two are bent. The chemical bonds between the two component parts of NaOS, Na and SO, for both 2A’ and 2A‘ bent structures have ionic character, although differing in degree. In all studies both double‐zeta‐plus polarization and triple‐zeta‐plus double polarization basis sets were used.

Imre G Csizmadia - One of the best experts on this subject based on the ideXlab platform.

  • a prelude to building mathematical models for polypeptide folding analysis on the conformational Potential Energy Hypersurface cross sections of n acetyl glycyl glycine n methylamide
    Canadian Journal of Chemistry, 2018
    Co-Authors: John Justine S Villar, Logine Negm, Anita Ragyanszki, David H Setiadi, Adrian Roy L Valdez, Bela Viskolcz, Imre G Csizmadia
    Abstract:

    Finding a relationship on how a three-dimensional protein folds from its linear amino acid chain gets more complex with increasing chain length, so working on a smaller peptide conformational problem can provide initial ideas on what are the main molecular forces and how these influence the folding process. Following the study of conformations of amino acid units entering the proteins to understand the secondary structure of small peptides, this paper proposes mathematical models for the several two-rotor cross-sections of the five-dimensional N-acetyl-glycyl-glycine-N′-methylamide Potential Energy Hypersurface (PEHS). These cross-sections are extracted along the first glycine subunit, with its coordinates fixed at the five Energy minima of the glycine diamide. The resulting mathematical models yield an average RMSE of 1.36 kJ mol−1 and an average R2 of 0.9923 with respect to Energy values obtained from DFT calculations. The minima geometries obtained from these models are also in good agreement with DFT-...

  • exploration of the four dimensional conformational Potential Energy Hypersurface of n acetyl l aspartic acid n methylamide with its internally hydrogen bonded side chain orientation
    Journal of Physical Chemistry A, 2002
    Co-Authors: Gregory A Chass, Andras Perczel, Odon Farkas, Ladislaus L Torday, Andras Varro, Julius Gy Papp, Imre G Csizmadia
    Abstract:

    Side-chain conformational Potential Energy Hypersurfaces have been generated and analyzed for each of the nine possible backbone conformers of N-acetyl-L-aspartic acid-N' methylamide. A total of 37 out of the 81 possible conformers were found and optimized at the B3LYP/6-31G(d) level of theory. The relative energies as well as the stabilization exerted by the side-chain on the backbone have been calculated, at this level of theory, for the 37 optimized conformers. Various backbone-backbone (N-H...O=C) and backbone-side-chain (N-H...O=C; N-H...OH) hydrogen bonds were analyzed. The appearance of the notoriously absent α L backbone conformer was attributed to such a backbone-side-chain (BB-SC) hydrogen bonds as well as a very unusual backbone-backbone (BB-BB) hydrogen bond.

  • peptide models 3 conformational Potential Energy Hypersurface of formyl l valinamide
    Journal of the American Chemical Society, 1993
    Co-Authors: Wladia Viviani, Jean Louis Rivail, Andras Perczel, Imre G Csizmadia
    Abstract:

    Out of the 21 legitimate minima of the 3D Ramachandran map, E=E(O,ψ,χ 1 ), the existing 20 conformations of formyl-L-valinamidehave been determined by ab-initio SCF-MO computations. In the gauche-side-chain conformations (χ 1 =60 o and χ 1 =300 o ), the Pattern of minima on the backbone Potential Energy surface, i.e. on the 2D Ramachandran map, E=E(O,ψ), is equivalent to the backbone conformation of the corresponding L-alanine derivative, which shows the absence of the α L and e L conformations. However, in the anti conformation (χ 1 =180 o ) an additional backbone conformation, the one labeled as δ L , has disappeared