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At least 397 records · Page 22
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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 transition probabilities for the OH Meinel system

An electric dipole moment function (EDMF) for the X 2Pi ground state of OH, based on the complete active-space self-consistent field plus a multireference singles-plus-double excitation configuration-interaction procedure (using an extended Slater basis) is reported. Two theoretical EDMFS are considered: the MCSCF (7)-SCEP EDMF of Werner et al., (1983) and a previously unpublished EDMF based on the MCSCF multireference CI(SD) procedure using a large Slater basis. The theoretical treatment follows that of Mies (1974), except that the Hill and Van Vleck (1928) approximation to intermediate coupling is used. This approximation is shown to be accurate to better than 5 percent for the six principal branches of the OH Meinel system.

Langhoff, S. R.↗
NASA NTRS1986

Theoretical studies of diatomic and triatomic systems containing the group IB atoms Cu, Ag, and Au

Selected portions of the ground state potential energy surfaces of the Cu3, Ag3, AgCu2, and AuCu2 trimers are studied at the single-reference singles plus doubles configuration interaction and couple pair functional levels correlating 33 electrons. The calculations use the effective core potentials of Hay and Wadt (1985) to replace the deep core levels. The Cu3 and Ag3 molecules are found to have 2B2 obtuse-angled ground states, with low-lying 2A1 acute-angled excited states. The AgCu2 and AuCu2 molecules have 2A1 acute-angled ground states. The Cu3 molecule has a smaller 3d population than Ag3, and Cu3 has a smaller s electron density in the open-shell orbital than does Ag3, in agreement with recent ESR experiments.

Walch, Stephen P.↗
NASA NTRS1986

The nature of the bonding in the transition metal trimers

The electronic structure of the transition metal (TM) trimers was studied by comparing the bonding in the Ca3, Sc3, and Cu3 molecules. The complete active space SCF/externally contracted configuration interaction (CI) ratio for the low-lying states of Sc3 and Sc3(+) and the SCF/CI ratio for Ca3 and Cu3 trimers, all for near equilateral triangle geometries, were calculated. In addition, vertical excitation energies for Cu3 were computed, leading to a new assignment of the upper state in the resonant two-photon ionization spectrum. Based on these studies, bonding in other TM trimers was discussed.

Walch, Stephen P.↗
NASA NTRS1986

Oscillator strengths for Ar VII, Ca IX and Fe XV

The excitation energies and oscillator strengths are calculated for electric-dipole-allowed and intercombination transitions between 3s2 1S, 3s3p(1,3)P0, 3p2 3P, 1D, 1S and 3s3d(1,3)D states in Ar VII, Ca IX, and Fe XV ions of the magnesium sequence. These states are represented by the fairly large configuration-interaction expansions. The calculations have been carried out in both LS and intermediate coupling schemes. The relativistic corrections have been included through the Breit-Pauli Hamiltonian. The results are compared with previous theoretical calculations and with measurements.

Tayal, S. S.↗
NASA NTRS1987

Theoretical spectroscopic parameters for the low-lying states of the second-row transition metal hydrides

A systematic analysis of the low-lying states of all of the second-row transition metal (TM) hydrides except CdH is reported. The calculations included the dominant relativistic contributions through the use of the relativistic effective core potentials of Hay and Wadt (1985). Electron correlation was incorporated, using single-plus-double configuration interaction, the coupled pair functional (CPF) formalism of Ahlrichs et al. (1985), and the Chong and Langhoff (1986) modified version of the CPF method. The spectroscopic parameters D(e), r(e), and mu(e) determined for the low-lying states are compared with the available experimental data and previous theoretical results. In contrast to the first-row TM hydrides studied earlier (Chong et al., 1986), the spectroscopic constants for the second-row TM hydrides were found to be much less sensitive to the level of correlation treatment.

Langhoff, Stephen R.↗
NASA NTRS1987

Effective collision strengths for electron impact excitations in S II

Electron impact collision strengths for forbidden, semiforbidden, and allowed transitions in S II calculated using the R-matrix method are presented. Configuration interaction wave functions are used to represent the six target states included in the calculation. At low impact energies the collision strengths are dominated by resonances for several transitions. The contribution from higher partial waves is obtained in the close-coupling approximation with exchange terms omitted. Results are presented for the effective collision strengths over a wide temperature range (5000-150,000 K) of astrophysical interest. The present results for the 4S(0) yields 2P(0) transition are 20-30 percent lower than previous calculations, while for the 4S(0) yields 2D(0) transition they are in good agreement. The results are approximately 30 percent higher than those of Ho and Henry (1983) for the 4S(0) yields 2P(0) transition at 80,000 K, and the difference between the two results increases with decreasing temperatures (under 80,000 K).

Tayal, S. S.↗
NASA NTRS1987

Full CI benchmark calculations on N2, NO, and O2 - A comparison of methods for describing multiple bonds

Full configuration interaction (CI) calculations on the ground states of N2, NO, and O2 using a DZP Gaussian basis are compared with single-reference SDCI and coupled pair approaches (CPF), as well as with CASSCF multireference CI approaches. The CASSCF/MRCI technique is found to describe multiple bonds as well as single bonds. Although the coupled pair functional approach gave chemical accuracy (1 kcal/mol) for bonds involving hydrogen, larger errors occur in the CPF approach for the multiple bonded systems considered here. CI studies on the 1Sigma(g +) state of N2, including all single, double, triple, and quadruple excitations show that triple excitations are very important for the multiple bond case, and accounts for most of the deficiency in the coupled pair functional methods.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1987

Mixed Cu-simple metal dimers and trimers - CuLi, CuLi2, CuNa, CuK, CuBe, CuBe2, Cu2Be, CuAl, and CuAl2

Theoretical studies of selected diatomic and triatomic molecules containing copper and the simple metals Li, Na, K, Be, and Al are presented, with emphasis on elucidating the nature of the bonding in mixed transition metal-simple metal systems. Large Gaussian basis sets are used in the diatomic calculations, and are used to calibrate the triatomic calculations, in which somewhat smaller Gaussian basis sets are employed. Electron correlation is incorporated using both the single-reference singles plus doubles configuration interaction and coupled pair functional methods. It is found that alkali atoms form very polar sigma bonds with copper, and that the ionicity increases with the inclusion of higher excitations because they improve the electron affinity of copper, which in turn allows a larger negative charge on copper. Aluminum is found to form stronger bonds than beryllium, since it does not have to undergo sp hybridization. Some of the trimers bond by forming three-center three-electron bonds. These multicenter bonds are quite strong even when compared to the two-electron bonds in the dimers or to other bonding mechanisms in the trimers.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1987

Oscillator strengths for S I, S II, and S III

A series of calculations for atomic data of various sulfur and oxygen ions is examined. Recent observations of the Io plasma torus obtained with the Voyager UV Spectrometer, the IUE satellite short wavelength spectrograph, and the rocket-borne faint object telescope are discussed. The calculation of oscillator strengths for S II, the P I sequence, S I, and S III in terms of configuration interaction effects is described. The derivation of orbital wave functions is considered. The use of the close coupling method to estimate collision strengths is studied. The accuracy of these calculations depends on: (1) the number of states used in the close coupling expansion; (2) resonance contributions to the thermally averaged collision strength; and (3) the quality of the target state wave functions. Tables of the derived oscillator strengths are presented.

Ho, Y. K.↗
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 NTRS1988

Core-core and core-valence correlation

The effect of (1s) core correlation on properties and energy separations was analyzed using full configuration-interaction (FCI) calculations. The Be 1 S - 1 P, the C 3 P - 5 S and CH+ 1 Sigma + or - 1 Pi separations, and CH+ spectroscopic constants, dipole moment and 1 Sigma + - 1 Pi transition dipole moment were studied. The results of the FCI calculations are compared to those obtained using approximate methods. In addition, the generation of atomic natural orbital (ANO) basis sets, as a method for contracting a primitive basis set for both valence and core correlation, is discussed. When both core-core and core-valence correlation are included in the calculation, no suitable truncated CI approach consistently reproduces the FCI, and contraction of the basis set is very difficult. If the (nearly constant) core-core correlation is eliminated, and only the core-valence correlation is included, CASSCF/MRCI approached reproduce the FCI results and basis set contraction is significantly easier.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1987

An initio calculations on C2, Si2, and SiC

Full-configuration-interaction (FCI) calculations have been performed for the Z 1Sigma(g)(+), a 3Pi(u), and b 3Sigma(g)(-) states of C2 and the X 3Sigma(g)(-) and A 3Pi(u) states of Si2 in valence double-zeta plus polarization one-particle basis sets. By carrying out the CASSCF multireference singles plus doubles CI (MRCI) procedure in a very large Gaussian basis set and correcting for errors with the FCI, the triplet-triplet separation in C2 is reproduced to within 47/cm. Analogous calculations on Si2 definitively determine the ground state to be 3Sigma(g)(-); the 3Pi(u) state is only 440 + or - 100/cm higher in energy. For the SiC molecule, which has never been observed spectroscopically but is believed to be an important component in carbon stars and to be present in interstellar space, the A 3Sigma(-) state is nearly 4000/cm above the X 3Pi ground state.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1987

Ab initio studies of low-lying 3Sigma(-), 3Pi, and 5Sigma(-) states of NH. I - Potential curves and dipole moment functions

Configuration interaction wave functions, potential energy curves, and dipole moment functions have been calculated for the four lowest 3Sigma(-) and the three lowest 3Pi states and 5Sigma(-) states of NH. The electronic wave functions were constructed to give a balanced description of valence-Rydberg interactions. Two repulsive states have been identified as important photodissociation pathways. Spectroscopic constants are presented for the bound states, and results are compared to other theoretical and experimental work. The possible predissociation of the A 3Pi state by the 1 5Sigma(-) state is discussed.

Goldfield, Evelyn M.↗
NASA NTRS1987

Theoretical study of the N2(+) Meinel system

Atomic natural orbital Gaussian basis sets and large complete-active-space self-consistent field/multireference configuration-interaction wave functions are used to determine the potential curves for the X 2Sigma(+) sub g and A 2Pi sub u states of N2(+). The dependence of the electronic transition moment between the two states on internuclear distance is accurately determined, and is shown to differ significantly from previous determinations. These theoretically determined lifetimes agree with experimental time-of-flight data within stated experimental error bars.

Langhoff, Stephen R.↗
NASA NTRS1988

Core-core and core-valence correlation

The effect of 1s core correlation on properties and energy separations are analyzed using full configuration-interaction (FCI) calculations. The Be1S - 1P, the C 3P - 5S,m and CH(+) 1Sigma(+) - 1Pi separations, and CH(+) spectroscopic constants, dipole moment, and 1Sigma(+) - 1Pi transition dipole moment have been studied. The results of the FCI calculations are compared to those obtained using approximate methods.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1988

Theoretical confirmation of a 4-Delta ground state for FeH

Large CAS SCF/multireference configuration interaction (MRCI) and modified coupled pair functional (MCPF) calculations are carried out for the 4-Delta and 6-Delta states of FeH using very large Gaussian basis sets. At the MCPF level, 3s and 3p correlation preferentially lowers the 4-Delta state by 0.10 eV. Adding this inner-shell correlation effect to our best CAS SCF/MRCI+Q valence treatment results in a 4-Delta-6 Delta separation of 0.16 eV. This supports the interpretation of the photodetachment spectra of Stevens et al. (1983) that places the 6-Delta state about 0.25 eV above the 4-Delta ground state.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1988

Electron density-sensitive line ratios for Fe XII in the solar corona

The collision strengths for electron-impact excitation of fine-structure transitions from the ground 3s23p3 configuration to the excited 3s3p4 configuration in Fe XII are presented. These are calculated by transforming the K-matrices obtained in a nonrelativistic R-matrix method to K-matrices in intermediate coupling using term-coupling coefficients which take into account the effect of breakdown of target LS coupling. Extensive configuration-interaction wave functions are used to represent the target states. The effective collision strengths are determined assuming a Maxwellian distribution of electron energies. Rydberg series of resonances converging to the excited state thresholds are explicitly included in the calculation. The relative level populations and emission-line strengths for Fe XII are calculated using the new atomic data presented in this paper and that published by Tayal, Henry, and Pradhan (1987) and Tayal and Henry (1986) over a wide range of electron densities and temperatures (1-2 x 10 to the 6th K). Present results for collision strengths and emission-line strengths are compared with the previous calculation of Flower (1977). Some significant differences are noted.

Tayal, S. S.↗