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At least 217 records · Page 12
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NASA NTRS1986

Theoretical study of the X 2Pi and A 2Sigma(+) states of CuO and CuS

Theoretical spectroscopic constants and dipole moments are determined for the X 2Pi and A 2Sigma(+) states of CuO and CuS, using extended Gaussian basis sets and incorporating correlation using both configuration interaction (CI) and coupled pair (CPF) methods. Relativistic corrections (Darwin plus mass velocity), included using first-order perturbation theory, are found to be relatively small. At the CPF level, significant configuration mixing with the 3d(9)4s(1) occupation occurs for both states, resulting in some bond shortening and a significant increase in the dissociation energy. The best spectroscopic parameters are in excellent agreement with experiment for both CuO and CuS. In analogy with CuO, CuS is predicted to have a yet unobserved 2Sigma(+) state near 10,000 per cm.

Langhoff, S. R.↗
NASA NTRS1986

Benchmark full configuration-interaction calculations on HF and NH2

Full configuration-interaction (FCI) calculations are performed at selected geometries for the 1-sigma(+) state of HF and the 2-B(1) and 2-A(1) states of NH2 using both DZ and DZP gaussian basis sets. Higher excitations become more important when the bonds are stretched and the self-consistent field (SCF) reference becomes a poorer zeroth-order description of the wave function. The complete active space SCF - multireference configuration-interaction (CASSCF-MRCI) procedure gives excellent agreement with the FCI potentials, especially when corrected with a multi-reference analog of the Davidson correction.

Bauschlicher, C. W., Jr.↗
NASA NTRS1986

Calculated potential surfaces for the reactions: O + N2 yields NO + N and N + O2 yields NO + O

Complete Active Space SCF/Contracted CI (CASSCF/CCI) calculations, using large Gaussian basis sets, are presented for selected portions of the potential surfaces for the reactions in the Zeldovich mechanism for the conversion of N2 to NO. The N + O2 reaction is exoergic by 32 kcal/mole and is computed to have an early barrier of 10.2 kcal/mole for the (sup 2)A(sup prime) surface and 18.0 kcal/mole for the (sup 4)A(sup prime) surface. The O + N2 reaction is endoergic by 75 kcal/mole. The (sup 3)A(sup double prime) surface is calculated to have a late barrier of 0.5 kcal/mole, while the (sup 3)A(sup prime) surface is calculated to have a late barrier of 14.4 kcal/mole.

Walch, Stephen P.↗
NASA NTRS1987

Potential surfaces for O atom-polymer reactions

Ab initio quantum chemistry methods are used to study the energetics of interactions of O atoms with organic compounds. Polyethylene (CH2)n has been chosen as the model system to study the interactions of O(3P) and O(1D) atoms with polymers. In particular, H abstraction is investigated and polyethylene is represented by a C3 (propane) oligomeric model. The gradient method, as implemented in the GRADSCF package of programs, is used to determine the geometries and energies of products and reactants. The saddle point, barrier geometry is determined by minimizing the squares of the gradients of the potential with respect to the internal coordinates. To correctly describe the change in bonding during the reaction at least a two configuration MCSCF (multiconfiguration self consistent field) or GVB (generalized valence bond) wave function has to be used. Basis sets include standard Pople and Dunning sets, however, increased with polarization functions and diffuse p functions on both the C and O atoms. The latter is important due to the O(-) character of the wave function at the saddle point and products. Normal modes and vibrational energy levels are given for the reactants, saddle points and products. Finally, quantitative energetics are obtained by implementing a small CAS (complete active space) approach followed by limited configuration interaction (CI) calculations. Comparisons are made with available experimental data.

Laskowski, B. C.↗
NASA NTRS1986

Studies of electron-molecule collisions - Applications to e-H2O

Elastic differential and momentum transfer cross sections for the elastic scattering of electrons by H2O are reported for collision energies from 2 to 20 eV. These fixed-nuclei static-exchange cross sections were obtained using the Schwinger variational approach. In these studies the exchange potential is directly evaluated and not approximated by local models. The calculated differential cross sections, obtained with a basis set expansion of the scattering wave function, agree well with available experimental data at intermediate and larger angles. As used here, the results cannot adequately describe the divergent cross sections at small angles. An interesting feature of the calculated cross sections, particularly at 15 and 20 eV, is their significant backward peaking. This peaking occurs in the experimentally inaccessible region beyond a scattering angle of 120 deg. The implication of this feature for the determination of momentum transfer cross sections is described.

Brescansin, L. M.↗
NASA NTRS1986

Benchmark full configuration-interaction calculations on H2O, F, and

Full configuration-interaction calculations are reported, and compared to other methods, for H2O at its equilibrium geometry and at two geometries with the H-O bonds stretched. Since the percentage of the self-consistent field (SCF) reference in the full configuration-interaction (FCI) wave function decreases greatly with the bond elongation, the accuracy of techniques based on a single reference do not compare well with the FCI results. However, the results from a complete active space SCF/multireference configuration-interaction (CASSCF/MRCI) treatment are in good agreement with the FCI. Correlation effects in F compared to Ne are far more similar than for Fcompared to Ne, despite F- and Ne being isoelectronic. Since the importance of higher than double excitations is more important for F- than F, a very high percentage of the correlation must be obtained to accurately compute the electron affinity. In a CASSCF/MRCI treatment the higher than quadruple excitations contribute 0.02 eV to the electron affinity (EA), even for modest basis sets.

Bauschlicher, C. W., Jr.↗
NASA NTRS1986

Theoretical study of the low-lying electronic states of ZnO and ZnS

Theoretical spectroscopic constants and dipole moments are determined for the 1 Sigma(+), 1,3 Pi, and 3 Sigma(+) states of ZnO and ZnS, using extended Gaussian basis sets and incorporating correlation using both configuration-interaction and coupled pair (CPF) methods. Relativistic corrections (Darwin plus mass velocity), included using first-order perturbation theory, are relatively small. At the CPF level, both ZnO and ZnS have 1 Sigma(+) ground states, with the 3 Pi state lying 209 and 2075/cm higher, respectively. The 3 Sigma(+) state lies about 1.5 eV higher in ZnO and 2.1 eV higher in ZnS. The 1,3 Pi states are relatively close together since the exchange splitting is small with the sigma electron localized on Zn and the pi electron on oxygen (or sulfur).

Bauschlicher, C. W., Jr.↗
NASA NTRS1986

Theoretical study of the X1Sigma(+) states of the alkali hydrides NaH-CsH

By means of near Hartree-Fock quality Slater basis sets, and the incorporation of electron correlation through the coupled-pair formalism, theoretical potentials are obtained for the X1Sigma(+) states of NaH, KH, and RbH. Electric dipole moment functions are given for NaH-RbH, as well as vibrationally averaged dipole moments, Einstein coefficients, and radiative lifetimes for the first 10 vibrational levels; an extensive study is made of the computational requirements for an accurate permanent dipole moment of KH.

Langhoff, Stephen R.↗
NASA NTRS1986

Stability and structure of metal clusters - Be(13) and Be(55)

Face-centered cubic (fcc) and hexagonally close-packed (hcp) structures are compared for Be(13) and Be(55) clusters. Both Be(13) and Be(55) prefer the fcc structure over the bulk hcp structure, but the energy difference per atom decreases for Be(55) relative to Be(13). The binding energy per atom, 1.3 eV for Be(55) and 0.8-0.9 eV for Be(13), reflects the greater total number of bonds in the larger cluster rather than a difference in bonding. The energies per bond are much more similar, in the range of 0.30-0.34 eV for both clusters. The size of the p-basis set used influences both stability and ionization potentials strongly.

Pettersson, Lars G. M.↗
NASA NTRS1986

A full CI treatment of the 1A1-3B1 separation in methylene

The accuracy of recent theoretical computations of the total energies and the adiabatic separation of the 1A1 and 3B1 states of CH2 is investigated on the basis of complete CI calculations using the double-zeta basis sets of Dunning (1970). The results are presented in a table and characterized in detail. The errors in the separation values are found to range from less than 0.01 kcal/mol for a CASSCF/MRSDCI calculation, to 0.38 kcal/mol for a Davidson-corrected SCF/SDCI calculation, to 14.17 kcal/mol for an uncorrected SCF calculation.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1986

Microwave spectrum and structure of bicyclo /1.1.1/ pentanone and comparison with several theoretical structures

The microwave spectra of five isotopic species of bicyclo (1.1.1) pentanone have been investigated. The rotational constants along with various centrifugal distortion constants for each species have been determined. From the rotational constants, a complete r(s) structure has been determined for the heavy atoms. Analysis of Stark effect measurements has shown the dipole moment to be along the a principal inertial axis with a magnitude of 3.164 (5) D. These results are compared with those obtained by four current theoretical methods: molecular mechanics (MM2), MNDO, and Hartree-Fock ab initio theory with STO-3G and 3-21G basis sets.

Mcrae, Glenn A.↗
NASA NTRS1987

On the 3d6 4s2 (5D) - 3d7 4s1 (5F) separation in Fe

Full CI calculations are used to calibrate SDSI treatment of the 5D-5F separation in the Fe atom. The (5s4p2d), (5s4p3d), and (5s4p2d1f) basis sets were used for the FCI calculations. The importance of the 3s and 3p correlation and the relativistic effects are studied at SDCI level. The role of polarization functions in computing atomic separations in transition metal atoms is discussed. It is observed that the FCI separation is slightly smaller than the SDCI results, and the SDCI data yield an adequate description of the 5D-5F separation with only 8 electrons correlated. It is concluded that the differential effect of higher excitations becomes more important when the 3s and 3p electrons are correlated.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1987

Calculated potential surfaces for the reactions - O + N2 - NO + N and N + O2 - NO + O

Complete active space SCF/contracted CI calculations using large Gaussian basis sets are presented for selected portions of the potential surfaces for reactions in the Zeldovich mechanism for the conversion of N2 to NO. The N + O2 reaction is exoergic by 32 kcal/mol and is computed to have an early barrier of 10.2 kcal/mol for the 2A-prime surface and 18.0 kcal/mol for the 4A-prime surface. The O + N2 reaction is endoergic by 75 kcal/mol. The 3A-double prime surface is calculated to have a late barrier of 0.5 kcal/mol, while the 3A-prime surface has a late barrier of 14.4 kcal/mol relative to NO + N. These results are significant for determining the physical and chemical conditions which aeroassisted orbital transfer vehicles will encounter while transferring between high and low altitude earth orbits.

Walch, Stephen P.↗
NASA NTRS1987

Accurate ab initio calculations which demonstrate a 3Pi(u) ground state for Al2

The computational requirements for accurately describing the spectroscopic constants for all three candidates for the ground state of Al2 are determined. Full CI(FCI) calculations are used to calibrate approximate methods of including the electron correlation. CASSCF/MRCI calculations which accurately reproduce the FCI results in the valence DZ + 2d Gaussian basis are carried to chemical accuracy by using extensive one-particle basis sets. The effect of 2s and 2p correlation and relativistic effects are considered as well as the valence 3s and 3p correlation. Several excited states are also considered. It is shown that the computed vibrational frequency of the (2) 3Pi(g) state and the Franck-Condon factors for the (1) 3Pi(u) - (2) 3Pi(g) transition are consistent with the spectrum obtained by Douglas et al. (1983) and Abe and Kolb (1983). It is therefore concluded that the ground state is 3Pi(u).

Bauschlicher, Charles W., Jr.↗
NASA NTRS1987

On the 1A1 - 3B1 separation in CH2 and SiH2

The 1A1 - 3B1 separation in CH2 and SiH2 has been computed using extended basis sets and CASSCF/SOCI wave functions. Using theoretical estimates for the effects of zero-point vibration yields T(0) values of 8.9 and -20.9 kcal/mol respectively, in excellent agreement with the experimental values of 9.02 and -21.0 kcal/mol. A corollary to the small zero-point vibrational contribution to the separation is that the symmetric stretching fundamental in CH2(3B1) must be near 3100/cm, much less than a recently suggested value of around 3400/cm. An accurate Te value for SiH2 establishes the ionization potential of the 1A1 state as 9.15 eV, the higher of two recent experimental values.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1987

Positive ions of the first- and second-row transition metal hydrides

Theoretical dissociation energies for the first- and second-row transition metal hydride positive ions are critically compared against recent experimental values obtained from ion beam reactive scattering methods. Theoretical spectroscopic parameters and dipole moments are presented for the ground and several low-lying excited states. The calculations employ large Gaussian basis sets and account for electron correlation using the single-reference single- and double-excitation configuration interaction and coupled-pair-functional methods. The Darwin and mass-velocity contributions to the relativistic energy are included in the all-electron calculations on the first-row systems using first-order perturbation theory, and in the second-row systems using the Hay and Wadt relativistic effective core potentials. The theoretical D(0) values for the second-row transition metal hydride positive ions should provide a critical measure of the experimental values, which are not as refined as many of those in the first transition row.

Pettersson, Lars G. M.↗
NASA NTRS1987

A high-precision ab initio determination of the equilibrium geometry and force field of HOC(+)

The results of an ab initio molecular orbital investigation of the isoformyl cation, HOC(+), shape are reported. The effects of expanding the basis set to near the Hartree-Fock limit and of electron correlation were examined, and the results indicate that near the Hartree-Fock limit the HOC(+) is linear. An analytic potential function is presented, from which the calculated rotational energies are only 0.03 percent different from the experimental values. This represents a nearly two orders of magnitude reduction in error from earlier work.

Defrees, D. J.↗
NASA NTRS1987

Effective core potential parameters for first- and second-row atoms

An improved effective core potential (ECP) technique is described and used to give ECP parameters for the atoms of the first two rows of the periodic table. A given basis set is parametrized which allows for a direct comparison with all-electron calculations. Extensive test calculations on first- and second-row molecules using the ECP have been performed, giving excellent agreement with the all-electron results at the SCF, CASSCF, and CI levels of treatment. Correlating and diffuse functions may be added without modifying the ECP parameters. The present ECP descriptions result in CPU time reductions of the order of 50 percent in addition to reduced disk storage.

Pettersson, Lars G. M.↗