Potential Energy Surfaces and Dynamics Calculations, 1981
A major question in quantum chemistry is: How do hydrogen molecules exchange hydrogen atoms? Many... more A major question in quantum chemistry is: How do hydrogen molecules exchange hydrogen atoms? Many have considered this query,1-18 but none have satisfactorily answered it. Bimolecular exchange has been studied with increasingly sophisticated methodology (large configuration interactions, double-zeta-plus-polarization basis sets).7 Geometries considered for the transition-state include: squares,1,2,3,7 kites,2,7 tetrahedra,2,3 chains,2,3,5,7,13 trapezoids,2,7 Y’s,7 T’s,6 and rhomboids.2,3,7 To date, the lowest calculated Ab initio bimolecular adiabatic activation energy is about 490 kJ/mol above 2H2.13 In contrast, the experimental Arrhenius activation energy corresponding to a second order bimolecular reaction is about 170 kJ/mol above 2H2.16,18
Ab initio enthalpy computations were carried out for over 40 gas-phase diamagnetic molecules (inc... more Ab initio enthalpy computations were carried out for over 40 gas-phase diamagnetic molecules (including 18 hydrocarbons). All employed optimized geometries, basis sets ranging from 4-31G to 6-311++G(2df,2pd), and a series of electron-correlation approximations (MP2, MP3, MP4SDQ, and MP4SDTQ, as well as CCD, CCSD, CCSD+T(CCSD), and several CCSDT versions). The energies of forming the various molecules from the nuclei and electrons at 0 K with no nuclear motion were calculated from experimental data and compared with the various ab initio values. The percentage difference between these experimental and ab initio values without correlation was found to be characteristic of each molecule regardless of the size of the basis set
A unified summary is presented of the mathematical approach developed by McDowell for employing p... more A unified summary is presented of the mathematical approach developed by McDowell for employing perturbation theory to correct for basis-set incompleteness in ab initio SCF calculations. Revised expressions for the corrections to the wavefunction both in terms of orbitals and spin-orbitals are presented with explicit incorporation of the spin variables. Employing H2O as an example, we show that this approach
A formulation of pseudopotential and effective‐potential theory is developed within the framework... more A formulation of pseudopotential and effective‐potential theory is developed within the framework of the Hartree–Fock formalism. It is shown that one‐electron pseudopotentials may be used for many‐valence‐electron atoms and molecules. An SCF computational procedure (the NOCOR method) is described that requires a minimal amount of parameterization. Sample calculations are performed on the four halogen atoms and ten dihalogen molecules. These results indicate that the ’’frozen‐core’’ approximation is quite accurate at least through the fourth row of the Periodic Table.
A new method for deriving anisotropic atomic dipole polarizability tensors for predicting molecul... more A new method for deriving anisotropic atomic dipole polarizability tensors for predicting molecular polarizabilities is presented. The procedure uses the relationships between atomic and molecular multipoles to rigorously derive the atomic polarizability tensors from quantum mechanically computed molecular dipole-quadrupole polarizabilities and their derivatives with respect to molecular deformations. The resulting atomic polarizabilities have been tested for the water molecule at
ABSTRACT The Phillips—Kleinman pseudopotential and the operator equivalence techniques for calcul... more ABSTRACT The Phillips—Kleinman pseudopotential and the operator equivalence techniques for calculating effective potentials are applied directly to the Dirac—Hartree—Fock calculation of molecular properties. The results based on both relativistic and nonrelativistic core functions are compared for the PbO molecule.
A set of nonbonded force field parameters consisting of atomic partial charges and van der Waals ... more A set of nonbonded force field parameters consisting of atomic partial charges and van der Waals parameters has been derived by fitting experimental data for a broad set of organic compounds. The compounds in the fit spanned 11 functional groups: alcohols, aldehydes, amides, amines, carboxylic acids, esters, ethers, N-heterocycles, hydrocarbons, ketones, and sulfur compounds. The data consist of 136 crystal structures, 34 sublimation energies, and 63 gas-phase dipole moments. The nonbonded potential function is of the “9-6” form and is used in our class II CFF force field. This paper describes the fitting procedure and the quality of the fit as measured by comparing computed and experimental gas-phase dipole moments and crystal lattice energies. As a further test, each crystal structure was optimized. We report the accuracy of the computed crystal structures, including results obtained with force fields derived using two different combination rules. A summary is presented of the accuracy and consistency ...
Potential Energy Surfaces and Dynamics Calculations, 1981
A major question in quantum chemistry is: How do hydrogen molecules exchange hydrogen atoms? Many... more A major question in quantum chemistry is: How do hydrogen molecules exchange hydrogen atoms? Many have considered this query,1-18 but none have satisfactorily answered it. Bimolecular exchange has been studied with increasingly sophisticated methodology (large configuration interactions, double-zeta-plus-polarization basis sets).7 Geometries considered for the transition-state include: squares,1,2,3,7 kites,2,7 tetrahedra,2,3 chains,2,3,5,7,13 trapezoids,2,7 Y’s,7 T’s,6 and rhomboids.2,3,7 To date, the lowest calculated Ab initio bimolecular adiabatic activation energy is about 490 kJ/mol above 2H2.13 In contrast, the experimental Arrhenius activation energy corresponding to a second order bimolecular reaction is about 170 kJ/mol above 2H2.16,18
Ab initio enthalpy computations were carried out for over 40 gas-phase diamagnetic molecules (inc... more Ab initio enthalpy computations were carried out for over 40 gas-phase diamagnetic molecules (including 18 hydrocarbons). All employed optimized geometries, basis sets ranging from 4-31G to 6-311++G(2df,2pd), and a series of electron-correlation approximations (MP2, MP3, MP4SDQ, and MP4SDTQ, as well as CCD, CCSD, CCSD+T(CCSD), and several CCSDT versions). The energies of forming the various molecules from the nuclei and electrons at 0 K with no nuclear motion were calculated from experimental data and compared with the various ab initio values. The percentage difference between these experimental and ab initio values without correlation was found to be characteristic of each molecule regardless of the size of the basis set
A unified summary is presented of the mathematical approach developed by McDowell for employing p... more A unified summary is presented of the mathematical approach developed by McDowell for employing perturbation theory to correct for basis-set incompleteness in ab initio SCF calculations. Revised expressions for the corrections to the wavefunction both in terms of orbitals and spin-orbitals are presented with explicit incorporation of the spin variables. Employing H2O as an example, we show that this approach
A formulation of pseudopotential and effective‐potential theory is developed within the framework... more A formulation of pseudopotential and effective‐potential theory is developed within the framework of the Hartree–Fock formalism. It is shown that one‐electron pseudopotentials may be used for many‐valence‐electron atoms and molecules. An SCF computational procedure (the NOCOR method) is described that requires a minimal amount of parameterization. Sample calculations are performed on the four halogen atoms and ten dihalogen molecules. These results indicate that the ’’frozen‐core’’ approximation is quite accurate at least through the fourth row of the Periodic Table.
A new method for deriving anisotropic atomic dipole polarizability tensors for predicting molecul... more A new method for deriving anisotropic atomic dipole polarizability tensors for predicting molecular polarizabilities is presented. The procedure uses the relationships between atomic and molecular multipoles to rigorously derive the atomic polarizability tensors from quantum mechanically computed molecular dipole-quadrupole polarizabilities and their derivatives with respect to molecular deformations. The resulting atomic polarizabilities have been tested for the water molecule at
ABSTRACT The Phillips—Kleinman pseudopotential and the operator equivalence techniques for calcul... more ABSTRACT The Phillips—Kleinman pseudopotential and the operator equivalence techniques for calculating effective potentials are applied directly to the Dirac—Hartree—Fock calculation of molecular properties. The results based on both relativistic and nonrelativistic core functions are compared for the PbO molecule.
A set of nonbonded force field parameters consisting of atomic partial charges and van der Waals ... more A set of nonbonded force field parameters consisting of atomic partial charges and van der Waals parameters has been derived by fitting experimental data for a broad set of organic compounds. The compounds in the fit spanned 11 functional groups: alcohols, aldehydes, amides, amines, carboxylic acids, esters, ethers, N-heterocycles, hydrocarbons, ketones, and sulfur compounds. The data consist of 136 crystal structures, 34 sublimation energies, and 63 gas-phase dipole moments. The nonbonded potential function is of the “9-6” form and is used in our class II CFF force field. This paper describes the fitting procedure and the quality of the fit as measured by comparing computed and experimental gas-phase dipole moments and crystal lattice energies. As a further test, each crystal structure was optimized. We report the accuracy of the computed crystal structures, including results obtained with force fields derived using two different combination rules. A summary is presented of the accuracy and consistency ...
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Papers by Carl Ewig