Engineering topics
Publications and source records attributed to Bauschlicher, C. W., Jr..
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.
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.
Reference is made to a study by Hay and Dunning (1976), who showed that, for most problems, it is not necessary to perform full FOCI (first order configuration interaction); instead, only that subset of configuration state functions (CSFs) that differ by only one or two spin orbitals from any of a list of reference configurations need be used. This is referred to as POLCI. Calculations of the D(e) of Cu2 using both SDCI (singles and doubles configuration interaction) and POLCI models are presented. It is found that the size-consistency problems associated with SDCI cause an error in D(e), whereas the POLCI results are in excellent agreement with experiment. It is proposed that the orbitals be classified as inactive and active and that the inactive-inactive correlation be eliminated to reduce the size-consistency error. In addition, it is recommended that SDCI treatments be applied with caution to systems having a large number of electrons.
Explore the source record for details and available documents.
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.
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.
Explore the source record for details and available documents.
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.
Full valence and first-order CI wave functions are invariant with respect to unitary transformations among the valence orbitals. We exploit this degree of freedom and show that by transforming the valence orbitals into a corresponding orbital basis, nonorthogonal transition moment calculations become an easily managed task. Sample full valence calculations on several states of O2(+) and OF are also presented.
It has been found that the convergence problems for many ill conditioned single-configuration SCF calculations arise from mixing among only a small number of orbitals. This orbital set includes the highest closed, the partially filled, and (possibly) a few of the lowest virtual orbitals. The fact that convergence problems can be, in very large measure, linked to a small orbital set is used to design a hybrid MCSCF procedure in which these orbitals are treated using a second-order MCSCF method, while other mixings are treated with a lower-order method which avoids the time consuming integral transformation. Tests on BeO show that the hybrid method yields convergence even when the simple lower-order treatment diverges. The method is expected to facilitate determination of MCSCF wave functions for large basis problems and for the construction of potential energy surfaces.
Using a double zeta plus polarization basis set of Slater orbitals, full valence MCSCF (FVMCSCF) calculations were performed for the low-lying states of MgO. For each state the FVMCSCF calculations were used to identify the important configurations which are then used in the MCSCF calculation and subsequently as references in a single and double excitation CI calculation. This approach is found to treat all states equivalently, with the maximum error in the computed transition energies and equilibrium bond lengths of 800/cm and approximately 0.03 A, respectively. The b 3 Sigma + state which has yet to be characterized experimentally is predicted to have a transition energy of approximately 8300/cm and a bond length of 1.79 A. A spectroscopic analysis of the potential curves indicates that their shapes are in quite reasonable agreement with the range of experimental results.
Calculations of the ground and low-lying singlet states of BeO are performed in order to gain an understanding of the techniques needed to treat the excited states of other, more complex, ionic molecules. The MCSCF and CI calculations are based on a Gaussian basis set of slightly better than double zeta plus polarization quality for single configuration descriptions of the states. The calculated X-A and X-B state separations are found to be in agreement with experimental measurements. The 1 Sigma - and 1 Delta states are predicted to lie approximately 40,000 kaysers above the ground state and are identified as the C and D states.The 2 1 Pi state is found to be approximately 15,000 kaysers and the 3 1 Sigma + state is found to be approximately 65,000 kaysers above the ground state.
The paper presents the multiconfiguration-self-consistent (MCSCF) and configuration state functions (CSF) for the low-lying electronic states of MgO. It was shown that simple description of these states was possible provided the 1 Sigma(+) states are individually optimized at the MCSCF level, noting that the 1(3 Sigma)(+) and 2(1 Sigma)(+) states which nominally result from the same electron occupation are separated energetically. The molecular orbitals obtained at this level of approximation should provide a useful starting point for extended configuration interaction calculations since they have been optimized for the particular states of interest.
The energies of the 3F (3d2 4s2), 5F (3d3 4s1) and 5D (3d4) states of Ti are investigated on the basis of MCSCF and CI calculations. Following SCF calculations using a basis set including the primitive set of Wachters (1970) and the diffuse d function of Hay (1977), four full-electron CI calculations involving the 3d and 4s electrons were performed to obtain the natural orbitals, and the full CI calculations were repeated. MCSCF calculations then performed using the most important configurations of the natural orbital basis are found to be in agreement with those of Dunning (1980), but to differ from CI results. CI(SD) calculations including single and double excitations from the MCSCF reference are found to be in agreement with the full CI but differ from experimental energy separations. Further calculations reveal that Dunning's MCSCF description is an oversimplication of the problem, and that the 5F-5D separation at the CI(SD) level is in good agreement with experiment for all basis sets but the 3F state is too low relative to the 5F and 5D states.
A previously reported multi-configuration self-consistent field (MCSCF) algorithm based on the generalized Brillouin theorem is extended in order to treat the excited states of polar molecules. In particular, the algorithm takes into account the proper treatment of nonorthogonality in the space of single excitations and invokes, when necessary, a constrained optimization procedure to prevent the variational collapse of excited states. In addition, a configuration selection scheme (suitable for use in conjunction with extended configuration interaction methods) is proposed for the MCSCF procedure. The algorithm is used to study the low-lying singlet states of BeO, a system which has not previously been studied using an MCSCF procedure. MCSCF wave functions are obtained for three 1 Sigma + and two 1 Pi states. The 1 Sigma + results are juxtaposed with comparable results for MgO in order to assess the generality of the description presented here.
The electronic structure of the 2 1A1 state of methylene is studied using multiconfiguration self-consistent-field and extended configuration interaction wavefunctions, as a prelude to a detailed investigation of the role of excited states in the chemistry of carbenes. Using selected one-electron properties as a guide, correlated wavefunctions obtained by different approaches are compared for the purpose of establishing qualitative differences between 1 1A1 and 2 1A1 states.
Electronic wavefunctions for 15 states of CaO arising (nominally) from six electron occupancies are calculated at the single configuration (sc) SCF level and analyzed qualitatively in terms of Mulliken populations and an approximate overlap criterion. The accuracy of the sc approximation is assessed and shown to be highly inadequate in several cases. Approximate Te's are extracted from these results when warranted by the quality of the sc wavefunctions. An improved two configuration wavefunction for the lowest 1 Sigma + state is also determined and discussed in detail.