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Walch, S. P.

Publications and source records attributed to Walch, S. P..

15 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1995

Minimum Energy Pathways for Chemical Reactions

Computed potential energy surfaces are often required for computation of such parameters as rate constants as a function of temperature, product branching ratios, and other detailed properties. We have found that computation of the stationary points/reaction pathways using CASSCF/derivative methods, followed by use of the internally contracted CI method to obtain accurate energetics, gives useful results for a number of chemically important systems. The talk will focus on a number of applications to reactions leading to NOx and soot formation in hydrocarbon combustion.

Walch, S. P.↗
NASA NTRS1986

Theoretical dipole moments for the first-row transition metal hydrides

Spectroscopic parameters (D sub e, r sub e, mu) are determined for the first-row transition metal hydrides using better than DZP basis sets at the modified coupled pair functional (MCPF) level. Extensive comparisons between MCPF and complete-active space self-consistent field (CASSCF)/MRCI calculations with natural orbital iterations, and studies with more extensive basis sets, show this level of treatment to supply an accurate and cost-effective treatment of these systems. For the transition metal hydrides, the bonding can arise from either the 3d(n)4s(2) or 3d(n + 1)4s(1) atomic asymptotes, or a mixture of both. Since the dipole moment arising from these two bonding mechanisms is very different, the dipole moment is found to be directly related to the 3d population. Thus, the magnitude of the dipole moments provides a sensitive test of the wave function, and gives insight into the nature of the bonding.

Chong, D. P.↗
NASA NTRS1986

Calculated ground state potential surface and excitation energies for the copper trimer

In the context of their relevance to catalysis and to materials science problems, transition metals and transition metal (TM) compounds are currently of considerable interest, and studies have been conducted of the copper trimer, Cu3. The present investigation is concerned with a study of the ground state surface and several groups of excited states in order to improve the understanding of the spectroscopy of Cu3. Differences of the current study from previous investigations are related to an employment of larger basis sets and a more extensive electron correlation. This was done with the objective to obtain a more accurate definition of the ground state surface. Features of the bonding in the copper dimer are considered to obtain a basis for an understanding of the copper trimer. Attention is given to calculational details, the ground state surface, and calculated vertical excitation energies. The results of SCF/SDCI calculations are reported for portions of the ground surface, for two groups of excited states, and for the ionization potential of Cu3.

Walch, S. P.↗
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 NTRS1985

Theoretical research program to study transition metal trimers and embedded clusters

Small transition metal clusters at a high level of approximation i.e. including all the valence electrons in the calculation and also including extensive electron correlation were studied. Perhaps the most useful end result of these studies is the qualitative information about the electronic structure of these small metal clusters, including the nature of the bonding. The electronic structure studies of the small clusters are directly applicable to problems in catalysis. From comparison of dimers, trimers and possibly higher clusters, it is possible to extrapolate the information obtained to provide insights into the electronic structure of bulk transition metals and their interaction with other atoms and molecules at both surface and interior locations.

Walch, S. P.↗
NASA NTRS1984

Theoretical research program to study transition metal trimers and embedded clusters

Small transition metal clusters were studied at a high level of approximation, including all the valence electrons in the calculation and extensive electron correlation, in order to understand the electronic structure of these small metal clusters. By comparison of dimers, trimers, and possibly higher clusters, the information obtained was used to provide insights into the electronic structure of bulk transition metals. Small metal clusters are currently of considerable experimental interest and some information is becomming available both from matrix electron spin resonance studies and from gas phase spectroscopy. Collaboration between theorists and experimentalists is thus expected to be especially profitable at this time since there is some experimental information which can serve to guide the theoretical work.

Walch, S. P.↗
NASA NTRS1984

The nature of chemisorbed oxygen on Ni(100) and Cu(100)

Model calculations indicate two low-lying states for the interaction of O with Ni and Cu(100). The oxide state is approximately 1 eV more stable and its R(perpendicular) for Ni(100) is consistent with the experimental value for Ni(100), indicating only one type of stable chemisorption bond. This agrees with XPS (X-ray photoemission spectroscopy) and EXAFS (extended X-ray absorption fine structure) data for the observed LEED (low energy electron diffraction) structures. Interpretations of EELS (electron energy loss spectroscopy) data in terms of a radical state forthe p(2x2) and the oxide state for the c(2x2) structure must be seriously questioned.

Bauschlicher, C. W., Jr.↗
NASA NTRS1983

Supplemental basis functions for the second transition row elements

It is noted that for molecular calculations, the basis sets presented by Huzinaga (1977) need to be augmented by (1) 5p functions to describe the 5s - 5p near degeneracy; (2) a diffuse 4d function to provide for a balanced description of the 5s2 4dn, 5s1 4d(n+1), and 4d(n+2) states of the atom; and (3) a set of 4f functions to correlate the 4d functions. Here, the diffuse 4d function is similar in function to the diffuse 3d function for the first transition row elements recommended by Hay (1977). A table is included giving the optimized values for the diffuse 4d, the 5p, and 4f (STO exponent) functions. The diffuse 4d function and the 5p functions are optimized at the SCF level on the basis of the 5s1 4d(n+1) state (except for Pd, which is optimized for the 4d10 state) and the 5s1 5p1 4dn state, respectively. The table also gives the energies and the atomic symmetries for each of the SCF calculations.

Walch, S. P.↗
NASA NTRS1983

Electron affinities of the alkali dimers - Na2, K2, and Rb2

Ab initio calculations on the ground states of the alkali dimers, Na2, K2, and Rb2, and their anions are reported. The calculations employ large Gaussian basis sets and account for nearly all of the valence correlation energy. The calculated atomic electron affinities are within 0.02 eV of experiment and the calculated adiabatic electron affinities for Na2, K2, and Rb2 are, respectively, 0.470, 0.512, and 0.513 eV.

Partridge, H.↗
NASA NTRS1983

An ab initio study of core-valence correlation

Especially in the cases of the first two columns of the periodic table, the inclusion of core-valence correlation in ab initio CI calculations yields a contraction of the atomic valence shell and improves both calculated atomic ionization potentials and atomic energy separations. For the alkali dimers Na2, K2, and Rb2, the presently calculated bond lengths are in excellent agreement with experiments when core-valence is included. In addition, the valence dissociation energies are accurate.

Partridge, H.↗
NASA NTRS1983

CASSCF/CI calculations for first row transition metal hydrides - The TiH(4-phi), VH(5-delta), CrH(6-sigma-plus), MnH(7-sigma-plus), FeH(4,6-delta) and NiH(2-delta) states

Calculations are performed for the predicted ground states of TiH(4-phi), VH(5-delta), CrH(6-sigma-plus), MnH(7-sigma-plus), Fett(4,6-delta) and NiH(2-delta). For FeH both the 6-delta and 4-delta states are studied, since both are likely candidates for the ground state. The ground state symmetries are predicted based on a combination of atomic coupling arguments and coupling of 4s(2)3d(n) and 4s(1)3d(n+1) terms in the molecular system. Electron correlation is included by a CASSCF/CI (SD) treatment. The CASSCF includes near-degeneracy effects, while correlation of the 3d electrons in included at the CI level.

Walch, S. P.↗
NASA NTRS1983

On the nature of the bonding in Cu2 - An ab initio viewpoint

The bonding in Cu2 has been calculatively investigated. A CASSCF calculation is performed where the two 4s electrons are distributed in all possible ways among the 4s and 4p orbitals. The CAS results have a 3d population of 9.99. The corresponding orbitals between the CAS core orbitals at r(e) and the atomic SCF core orbitals show an overlap of 13.9992 out of a possible 14, with all of the change in sigma space. Thus, the CAS results show virtually no change in the d orbitals relative to the atom and can be considered as showing only a single bond. The reduction in 3d population, the decrease in r(e), and the increase in D(e) upon correlating the 3d shell indicates that the 3d electrons are slightly involved in the bonding, but there is no indication of the formation of a triple bond.

Bauschlicher, C. W., Jr.↗
NASA NTRS1982

Ab initio calculation of the X 1 Sigma + state of CsH

Stevens et al. (1981) considered CsH as a two electron problem, but simulated the Cs core electrons by an empirical pseudopotential following the work by Bardsley (1970). These potentials, since they are derived empirically, include the relativistic effects and the atomic core-valence interactions. However, molecular core-core interactions are not accounted for. Stevens et al. obtained an R(e) value which is too small. This result was attributed to uncorrected core (proton)-core interactions in the molecule. The present investigation is concerned with a resolution of the discrepancies between the results of earlier studies. The X 1 Sigma + ground state of CsH is recalculated. The calculation employs a nine valence electron relativistic effective core potential (RECP) for Cs. The bonding in CsH is found to involve a Cs(6s)-H(1s) bond but with a significant ionic (Cs+H-) component.

Laskowski, B. C.↗
NASA NTRS1982

On correlation in the first row transition metal atoms

CI (SD) calculations using both SCF and MCSCF references have been performed for the first-row transition metal atoms. Basis sets as large as 84 STO's have been used. At the highest level of calculation, good agreement with experiment is found. If relativistic effects are considered, the agreement is somewhat poorer. This aspect, as well as the trends and results, is discussed.

Bauschlicher, C. W., Jr.↗