Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “basis sets”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 577 records · Page 32
All sourcesNASA NTRSDOE OSTI
NASA NTRS1988

Oscillator strengths for Si II

New accurate results are presented for absorption oscillator strengths of Si II resonance lines. In these ab initio calculations, an extensive basis set of 67 electronic configuration are employed, including fine-structure states and relativistic effects in the Breit-Pauli approximation. For the strong Si II lines, good agreement is found with previous LS-coupling f-values of Dufton et al. (1983). Oscillator strengths of selected weaker lines disagree in some cases with empirically determined f-values by Shull, Snow, and York (1981) and by Van Buren (1986). The theoretical results are combined with curve-of-growth analyses of Copernicus and IUE data to yield a list of recommended Si II oscillator strengths consistent with the errors in both theoretical and observational analyses. These f-values have been used (Van Steenberg and Shull) for a Galactic survey of interstellar silicon abundances with the International Ultraviolet Explorer satellite.

Luo, Ding↗
NASA NTRS1988

On the dissociation energies and bonding in NiCO(+) and TiCO(+)

Ab initio calculations on the NiCO(+) and TiCO(+) molecules are carried out using large Gaussian basis sets and extensive treatment of electron correlation. The NiCO(+) molecule is found to have a 2Sigma(+) ground state with a 2Delta state only 2.1 kcal/mole higher in energy. The Ni(+)-CO structure is about 11 and 28 kcal/mole more stable than the Ni(+)-OC and the T-shaped structures, respectively.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1988

On the interpretation of the photoelectron spectrum of NiCO(-)

Ab initio calculations have been carried out for the X1Sigma(+), a3Delta, b3Delta(+), and c3Pi states of NiCO, using a large Gaussian basis set and an MCPF treatment of electron correlation. The calculated dissociation energies, De (kcal/mol), are 30.1, 11.5, 10.1, and 4.3 respectively. An analysis of the relative cross sections for photodetachment indicates the the lower energy feature in the photoelectron spectrum of NiCO(-) results primarily from bound-free transitions to the c3Pi state, with some contribution from bound-bound transitions to both the b3Sigma(+) and c3Pi states.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1989

Ordering of the O-O stretching vibrational frequencies in ozone

The ordering of nu1 and nu3 for O3 is incorrectly predicted by most theoretical methods, including some very high level methods. The first systematic electron correlation method based on one-reference configuration to solve this problem is the coupled cluster single and double excitation method. However, a relatively large basis set, triple zeta plus double polarization is required. Comparison with other theoretical methods is made.

Scuseria, Gustavo E.↗
NASA NTRS1989

Theoretical studies of the transition metal-carbonyl systems MCO and M(CO)2, M = Ti, Sc, and V

Large Gaussian basis sets and an electron correlation treatment are used in ab initio calculations on the transition metal-carbonyl systems MCO and N(CO)2 for M = Ti, Sc, and V. The results show that high-spin ground states are favored for the monocarbonyl molecules, and that for the dicarbonyl molecules there is a competition between high, intermediate, and low spin states which are very close in energy. Dissociation energies of 0.62 eV for Ti-CO and 1.02 for Ti(CO)2 are found, relative to the ground state Ti atomic asymptote and CO 1Sigma(+). The binding energy per carbonyl for the three metal atoms is shown to be significantly lower for the dicarbonyl than for the the monocarbonyl molecules.

Barnes, Leslie A.↗
NASA NTRS1989

Theoretical characterization of selected regions of the ground state potential surface of N2H2

The results of CASSCF/multireference contracted CI calculations with large atomic natural orbital basis sets are presented for selected regions of the N2H2 potential surface. The calculations focused on the addition of an H atom to HN2. This process involves four channels. Approach of the H atom along the HN bond leads to N2 + H2 with no barrier (H abstraction), while approach of the H atom to the other three positions leads to trans-HNNH, cis-HNNH, and NNH2, which are stable minima. These calculations provide the most accurate energetics for these three stable species obtained to date. Combining the results of these calculations with previous results for HN2 provides accurate thermochemical data on these important molecules.

Walch, Stephen P.↗
NASA NTRS1989

Bonding in zerovalent Ni compounds - NiN2 and Ni(N2)4 compared with NiCO and Ni(CO)4

Calculations are carried out on NiN2, which may be considered a prototypical metal surface-ligand system. A large Gaussian basis set and an MCPF treatment of electron correlation are used. Consideration is also given to the 2Sigma(+) states of NiN2(-), NiCO(-), and NiN2(+), the low-lying 2Delta and 2Pi states of NiN2(+), and the 1A1 states of Ni(CO)4 and Ni(N2)4.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1989

Lowest ionization potentials of Al2

Potential curves for the lowest two electronic states, X 2Sigma(+)g and A 2Pi(u), of Al2(+) were computed using complete active space SCF/multireference CI wave functions and large Gaussian basis sets. The lowest observable vertical ionization potential, to Al2(+) X 2Sigma(+)g, of the Al2 X 3 Pi(u) ground state is calculated to occur around 6.1 eV, in excellent agreement with the experimental range of 6.0 to 6.42 eV obtained in recent cluster ionization studies by Cox and co-workers. The second vertical ionization potential, to Al2(+) A 2Pi(u), occurs near 6.4 eV, also within the experimental range. The adiabatic IP of 5.90 eV is in good agreement with the value of 5.8 to 6.1 eV deduced by Hanley and co-workers from the difference in thresholds between collision induced dissociation processes of Al3(+). The computed IP values are somewhat larger than those deduced from branching ratios in cluster fragmentation experiments by Jarrold and co-workers. The observation of an ionization threshold below 6.42 eV is shown to be incompatible with an Al2 ground electronic state assignment of 3Sigma(-)g, but the separation between the two lowest states of Al2 is so small that it is likely that both are populated in the experiments, so that this does not provide unambiguous support for the recent theoretical assignment of the ground state as 3Pi(u).

Bauschlicher, Charles W., Jr.↗
NASA NTRS1989

Theoretical characterization of the potential energy surface for H + O2 yields HO2(asterisk) yields HO + O. II - The potential for H atom exchange in HO2

The results of CASSCF multireference contracted CI calculations with large ANO basis sets are presented for the exchange region of the HO2 potential-energy surface. The saddle point for H atom exchange is about 13 kcal/mol below the energy of H + O2; therefore, this region of the surface should be accessible during H + O2 recombination and methathesis reactions.

Walch, Stephen P.↗
NASA NTRS1989

A high level Ab initio study of the anionic hydrogen-bonded complexes FH-CN-, FH-NC-, H2O-CN- and H2O-NC-

HF, H2O, CN- and their hydrogen-bonded complexes were studied using state-of-the-art ab initio quantum mechanical methods. A large Gaussian one particle basis set consisting of triple zeta plus double polarization plus diffuse s and p functions (TZ2P + diffuse) was used. The theoretical methods employed include self consistent field, second order Moller-Plesset perturbation theory, singles and doubles configuration interaction theory and the singles and doubles coupled cluster approach. The FH-CN- and FH-NC- and H2O-CN-, H2O-NC- pairs of complexes are found to be essentially isoenergetic. The first pair of complexes are predicted to be bound by approx. 24 kcal/mole and the latter pair bound by approximately 15 kcal/mole. The ab initio binding energies are in good agreement with the experimental values. The two being shorter than the analogous C-N hydrogen bond. The infrared (IR) spectra of the two pairs of complexes are also very similar, though a severe perturbation of the potential energy surface by proton exchange means that the accurate prediction of the band center of the most intense IR mode requires a high level of electronic structure theory as well as a complete treatment of anharmonic effects. The bonding of anionic hydrogen-bonded complexes is discussed and contrasted with that of neutral hydrogen-bonded complexes.

Lee, Timothy J.↗
NASA NTRS1989

Theoretical investigations of the structures and binding energies of Be(sub n) and Mg(sub n) (n = 3-5) clusters

Researchers determined the equilibrium geometries and binding energies of Be and Mg trimers, tetramers and pentamers using single and double excitation coupled cluster (CCSD) and complete active space self-consistent-field (CASSCF) multireference configuration interaction (MRCI) wave functions in conjunction with extended atomic basis sets. Best estimates of the cluster binding energies are 24, 83 and 110 kcal/mole for Be3, Be4 and Be5; and 9, 31 and 41 kcal/mole for Mg3, Mg4 and Mg5, respectively. A comparison of the MRCI and CCSD results shows that even the best single-reference approach (limited to single and double excitations) is not capable of quantitative accuracy in determining the binding energies of Be and Mg clusters.

Lee, Timothy J.↗
NASA NTRS1990

Theoretical investigation of gas-surface interactions

A Dirac-Hartree-Fock code was developed for polyatomic molecules. The program uses integrals over symmetry-adapted real spherical harmonic Gaussian basis functions generated by a modification of the MOLECULE integrals program. A single Gaussian function is used for the nuclear charge distribution, to ensure proper boundary conditions at the nuclei. The Gaussian primitive functions are chosen to satisfy the kinetic balance condition. However, contracted functions which do not necessarily satisfy this condition may be used. The Fock matrix is constructed in the scalar basis and transformed to a jj-coupled 2-spinor basis before diagonalization. The program was tested against numerical results for atoms with a Gaussian nucleus and diatomic molecules with point nuclei. The energies converge on the numerical values as the basis set size is increased. Full use of molecular symmetry (restricted to D sub 2h and subgroups) is yet to be implemented.

Dyall, Kenneth G.↗
NASA NTRS1989

On the electronic structure and spectrum of CuF2 and CuCl2

Results are reported from SDCI and CPF ab initio computations for the ground and low-lying states of CuF2 and CuCl2. A large Gaussian basis set with functions up to the g level is employed, and the results are presented in tables and characterized in detail. Particular attention is given to the bond distances and force constants for the ligand-field states and the bond distances for the charge-transfer states. Good agreement is obtained between the CPF and SDCI values and with published experimental data.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1989

Theoretical study of the dipole moment function of OH(X2Pi)

A theoretical study of the sensitivity of the dipole moment function (DMF) of the X2Pi ground state of OH to basis-set saturation and to refinements in the correlation treatment is presented. Emphasis is placed on determining the slope of the DMF at r(e) and the r value at which the maximum occurs. Consideration is given to the effect of oxygen polarization functions up through h type, expansion of the active orbital space to include the O 3d-delta orbital, the effect of higher excitations using the averaged coupled-pair functional method, and the effect of evaluating the dipole moment as an energy derivative rather than as an expectation value. The theoretical DMFs obtained here, which should be the most accurate to date, differ markedly from an empirical DMF of Turnbull and Lowe that is based on experimentally derived intensity ratios. The theoretical DMFs agree better with a recently published DMF of Nelson et al., but suggest that this empirical DMF is also inaccurate for r greater than 2.3 a0.

Langhoff, Stephen R.↗
NASA NTRS1989

The application of ab initio calculations to molecular spectroscopy

The state of the art in ab initio molecular structure calculations is reviewed with an emphasis on recent developments, such as full configuration-interaction benchmark calculations and atomic natural orbital basis sets. It is found that new developments in methodology, combined with improvements in computer hardware, are leading to unprecedented accuracy in solving problems in spectroscopy.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1989

The application of ab initio calculations to molecular spectroscopy

The state of the art in ab initio molecular structure calculations is reviewed, with an emphasis on recent developments such as full configuration-interaction benchmark calculations and atomic natural orbital basis sets. It is shown that new developments in methodology combined with improvements in computer hardware are leading to unprecedented accuracy in solving problems in spectroscopy.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1990

Theoretical investigations of the structures and binding energies of Be(n) and Mg(n) (n = 3-5) clusters

The equilibrium geometries and binding energies of Be and Mg trimers, tetramers and pentamers have been determined using single and double excitation coupled cluster (CCSD) and complete active space self-consistent-field (CASSCF) multireference configuration interaction (MRCI) wave functions in conjunction with extended atomic basis sets. The best estimates of the cluster binding energies are 24, 83, and 110 kcal/mol for Be3, Be4, and Be5; and 9, 31, and 41 kcal/mol for Mg3, Mg4, and Mg5, respectively. A comparison of the MRCI and CCSD results shows that even the best single-reference approach (limited to single and double excitations) is not capable of quantitative accuracy in determining the binding energies of Be and Mg clusters.

Lee, Timothy J.↗