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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.

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At least 109 records · Page 6
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NASA NTRS1984

Theoretical study of electron correlation effects in transition metal dimers

Introduction of partially localized orbitals is shown to reduce the number of terms needed to describe the bonding in transition metal clusters. Using this formalism, it is possible to compute the various intra- and inter-atomic electron correlation contributions to the bond energy. Calculations demonstrate the relative importance of several kinds of electron correlation terms involving the 3p, 3d, and 4s electrons. Improved interaction potentials are obtained for the dimers V(2) and Cr(2) when additional correlation is added to the CAS SCF results of Walch, Bauschlicher, Roos, and Nelin (1983).

Das, G. P.↗
NASA NTRS1985

Calculation of pressure broadening parameters for the CO-He system at low temperatures

Theoretical pressure broadening parameters were computed for the 0-1 and 1-2 rotational transitions of CO in He at very low temperatures and compared with the recent experimental measurements at 4.2 K. The interaction potential was taken from extensive SCF-CI calculations, molecular collision dynamics were described by essentially exact converged close coupling calculations, and pressure broadening cross sections were obtained from the collisional S matrices within the accurate Fano-Ben Reuven framework. Resonances at low collision energies give rise to an increase in the thermally averaged cross sections at low temperatures. Although previous calculations for this system at higher temperatures (77-300 K) were in good accord with experiment, at 4.2 K predicted values are about two times larger than experiment; possible sources of this discrepancy are discussed.

Green, S.↗
NASA NTRS1986

Theoretical research program to study chemical reactions in AOTV bow shock tubes

Progress in the development of computational methods for the characterization of chemical reactions in aerobraking orbit transfer vehicle (AOTV) propulsive flows is reported. Two main areas of code development were undertaken: (1) the implementation of CASSCF (complete active space self-consistent field) and SCF (self-consistent field) analytical first derivatives on the CRAY X-MP; and (2) the installation of the complete set of electronic structure codes on the CRAY 2. In the area of application calculations the main effort was devoted to performing full configuration-interaction calculations and using these results to benchmark other methods. Preprints describing some of the systems studied are included.

Taylor, P.↗
NASA NTRS1986

Integral processing in beyond-Hartree-Fock calculations

The increasing rate at which improvements in processing capacity outstrip improvements in input/output performance of large computers has led to recent attempts to bypass generation of a disk-based integral file. The direct self-consistent field (SCF) method of Almlof and co-workers represents a very successful implementation of this approach. This paper is concerned with the extension of this general approach to configuration interaction (CI) and multiconfiguration-self-consistent field (MCSCF) calculations. After a discussion of the particular types of molecular orbital (MO) integrals for which -- at least for most current generation machines -- disk-based storage seems unavoidable, it is shown how all the necessary integrals can be obtained as matrix elements of Coulomb and exchange operators that can be calculated using a direct approach. Computational implementations of such a scheme are discussed.

Taylor, P. R.↗
NASA NTRS1986

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

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 F- compared 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 NTRS1985

On 3d bonding in the transition metal trimers - The electronic structure of equilateral triangle Ca3, Sc3, Sc3(+), and Ti3(+)

It is pointed out that transition metals and transition metal (TM) compounds are currently of considerable interest because of their relevance to catalysis and to materials science problems such as hydrogen embrittlement and crack propagation in metals. The present paper is concerned with complete active space Self-Consistent Field (SCF) externally contracted configuration interaction (CASSCF/CCI) calculations for the low-lying states of Sc3 and Sc3(+). A comparison is conducted regarding the bonding in the Ca3, Sc3, and Cu3 molecules. This comparison makes it possible to predict general trends for the TM trimers. Attention is given to the qualitative features of the bonding in the TM trimers, the basis sets and other technical details of the calculations, the calculated results for Sc3 and Sc3(+), and conclusions from this work.

Walch, S. P.↗
NASA NTRS1986

The nature of the bonding in XCO for X = Fe, Ni, and Cu

The bonding in the (5,3)Sigma - and 3Delta states of FeCO, the (3,1)Sigma + , 3Delta, and 3Pi states of NiCO, and the 2Sigma + state of CuCO are analyzed using the constrained-space orbital-variation (CSOV) technique for both (CASSCF) and SCF wave functions. The bonding is discussed in terms of sigma repulsion between the metals 4s and the CO 5-sigma and CO-to-metal sigma donation when there is an empty or partly occupied d-sigma orbital and metal-to-CO 2pi(asterisk) backdonation. The bonding is compared for the different metals and between the different states.

Bauschlicher, C. W., Jr.↗
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 electric dipole moment function of the ClO molecule

The potential energy function and electric dipole moment function (EDMF) are computed for ClO X 2Pi using several different techniques to include electron correlation. The EDMF is used to compute Einstein coefficients, vibrational lifetimes, and dipole moments in higher vibrational levels. The band strength of the 1-0 fundamental transition is computed to be 12 + or - 2 per sq cm atm determined from infrared heterodyne spectroscopy. The theoretical methods used include SCF, CASSCF, multireference singles plus doubles configuration interaction (MRCI) and contracted CI, coupled pair functional (CPF), and a modified version of the CPF method. The results obtained using the different methods are critically compared.

Pettersson, L. 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 NTRS1987

Structure, vibrational spectrum, and energetics of the CH5(+) ion - A theoretical investigation

Analytic derivative techniques are utilized to determine molecular geometries, vibration spectra, molecular force field, proton affinity, and energetics of clustering of protonated methane. It is observed that the geometries are gradient optimized, while the harmonic force fields and IR intensities are also determined analytically at the SCF level. It is determined that the frequency of the torsional model for rotation of the H2 about the pseudothreefold axis of the CH3(+) group is extremely low, and the proton affinity of CH4 is calculated as 129.0 kcal/mol.

Komornicki, Andrew↗
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.↗
NASA NTRS1987

Small Al clusters. II - Structure and binding in Al(n) (n = 2-6, 13)

The structure and stability of aluminum clusters containing up to six atoms have been studied using correlated wave functions and extended basis sets. The lowest energy structure is planar for Al4 and Al5, but three dimensional for Al6. The icosahedral, hcp, fcc, and two planar structures of Al13 were considered at the SCF level. The lowest energy structure is the icosahedron, but the planar structures are fairly low lying even in this case. A simplified description using two- and three-body interaction potentials is found to agree well with the ab initio structures and binding energies.

Pettersson, Lars G. M.↗
NASA NTRS1987

Accurate ab initio calculations for the ground states of N2, O2 and F2

Spectroscopic constants and dissociation energies for the ground states of N2, O2, and F2 determined at the CAS SCF MRCI correlation level are in excellent agreement with experiment when very large primitive valence and polarization one-particle Gaussian basis sets are employed. The dissociation energy (De) for N2 is larger than experiment unless the 2s electrons are correlated. The basis set requirement for an accurate determination of De is found to increase with the degree of multiple-bond character in the molecule.

Langhoff, Stephen R.↗
NASA NTRS1988

A computational study of the TiO2 molecule

A computational investigation of the ground 1A1 and lowest energy B2 states of the titanium dioxide molecule has been carried out. The treatment utilized SCF calculations in an extended basis followed by a CI treatment for each geometry. The ground state geometry agrees well with experiment, while the agreement between the computed vibrational frequency nu1 and the experimental value for a matrix isolated TiO2 is less satisfactory. Population analysis for the ground state indicates less than one excess electron on each oxygen atom. The first excited state has a linear geometry and the singlet and triplet are essentially degenerate.

Ramana, M. V.↗
NASA NTRS1987

How important is correlation in the description of the Ni-H2O interaction

Using very large basis sets, which can accurately describe the polarizability of Ni and H2O and the dipole moment of H2O, the dipole-induced-dipole interaction is shown to be more than 50 percent of the about 4.8 kcal/mole bonding. Dispersion is found to account for the remainder of the bonding; this is significantly less than in previous estimates. The superposition error at the SCF level is essentially zero, and only 0.5 kcal/mole at the SDCI level, and the binding energy has been corrected for this error.

Bauschlicher, Charles W., Jr.↗
NASA NTRS1988

Theoretical study of the scandium and yttrium halides

The X1Sigma(+) ground states and a3Delta excited states of the diatomic halides of Sc and Y are characterized theoretically, using the SDCI coupled-pair functional method and the state-averaged CASSCF method to determine the spectroscopic constants and related properties. The techniques employed are discussed, and the results are presented in extensive tables. The dissociation energies are given as D0 = 6.00 eV for ScF, 4.55 eV for ScCl, 3.90 eV for ScBr, 6.72 eV for YF, 5.36 eV for YCl, and 4.74 eV for YBr.

Langhoff, Stephen R.↗