A density matrix approach to multiconfiguration calculations
Determination of density matrix components for multiconfiguration wave functions and Hamiltonian interaction matrices
Harriman, J. E.↗
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A density matrix approach to multiconfiguration calculations
Density matrix components for multiconfiguration wave functions, constructing N electron /spin free/ Hamiltonian configuration interaction matrix
Harriman, J. E.↗
Collision strengths for the intercombination lines of S III
The collision strengths of intercombination lines of S III ions observed in the Io torus are calculated on the basis of data gathered by Voyager instruments, the IUE and the Faint Object Telescope. The doubly ionized ions are excited by electron impacts and move from the 3P2P(3) ground state to excited 3s3P3(1)PO, (1)DO, 3p3d(1)PO, and 3p4s(1)PO states. Excitational cross sections are calculated with a two-state close-coupling approximation for the energy range up to 1,000,000 K, with target states being represented by multiconfiguration interaction wave functions. The close-coupling approximation regarded as an accurate technique because of the dominance of the Coulomb factor in the ionized state. The wave function calculations are estimated to yield values accurate to within 30 percent. Further work is need to quantify the magnitude of error introduced by resonance contributions to the collision strengths.
Ho, Y. K.↗
Quadrupole moment of CO, N2, and NO/+/
Evaluation of quadrupole moments obtained for CO, N2, and NO(+) with multiconfiguration self-consistent-field wave functions chosen by the optimized valence configuration (OVC) approach of Wahl and Das (1972). The quadrupole moments thus obtained are compared with both experimental and Hartree-Fock values. The Hartree-Fock results are found to be not very accurate for N2, especially if vibrational averages over vibrational levels greater than 1 or 2 are to be taken. The agreement between the OVC quadrupole moment and the experimental values is found to be best for CO.
Billingsley, F. P., II↗
Dipole moments and transition probabilities of the a 3Sigma(+)g - b 3Sigma(+)u system of molecular hydrogen
Multiconfiguration variational calculations of the electronic wave functions of the a 3Sigma(+)g and b 3Sigma(+)u states of molecular hydrogen are presented, and the electric dipole transition moment between them (of interest in connection with stellar atmospheres and the UV spectrum of the Jovian planets) is obtained. The dipole moment is used to calculate the probabilities of radiative transitions from the discrete vibrational levels of the a 3Sigma(+)g state to the vibrational continuum of the repulsive b 3Sigma(+)u state as functions of the wavelength of the emitted photons. The total transition probabilities and radiative lifetimes of the levels v prime = 0-20 are presented.
Guberman, S.↗
Hydrogen atom abstraction from aldehydes - OH + H2CO and O + H2CO
The essential features of the potential energy surfaces governing hydrogen abstraction from formaldehyde by oxygen atom and hydroxyl radical have been characterized with ab inito multiconfiguration Hartree-Fock (MCHF) and configuration interaction (CI) wave functions. The results are consistent with a very small activation energy for the OH + H2CO reaction, and an activation energy of a few kcal/mol for the O + H2CO reaction. In the transition state structure of both systems, the attacking oxygen atom is nearly collinear with the attacked CH bond.
Dupuis, M.↗
Potential surfaces for O atom-polymer reactions
Ab initio quantum chemistry methods are used to study the energetics of interactions of O atoms with organic compounds. Polyethylene (CH2)n has been chosen as the model system to study the interactions of O(3P) and O(1D) atoms with polymers. In particular, H abstraction is investigated and polyethylene is represented by a C3 (propane) oligomeric model. The gradient method, as implemented in the GRADSCF package of programs, is used to determine the geometries and energies of products and reactants. The saddle point, barrier geometry is determined by minimizing the squares of the gradients of the potential with respect to the internal coordinates. To correctly describe the change in bonding during the reaction at least a two configuration MCSCF (multiconfiguration self consistent field) or GVB (generalized valence bond) wave function has to be used. Basis sets include standard Pople and Dunning sets, however, increased with polarization functions and diffuse p functions on both the C and O atoms. The latter is important due to the O(-) character of the wave function at the saddle point and products. Normal modes and vibrational energy levels are given for the reactants, saddle points and products. Finally, quantitative energetics are obtained by implementing a small CAS (complete active space) approach followed by limited configuration interaction (CI) calculations. Comparisons are made with available experimental data.
Laskowski, B. C.↗
Cross Sections for Electron Impact Excitation of Astrophysically Abundant Atoms and Ions
Electron collisional excitation rates and transition probabilities are important for computing electron temperatures and densities, ionization equilibria, and for deriving elemental abundances from emission lines formed in the collisional and photoionized astrophysical plasmas. Accurate representation of target wave functions that properly account for the important correlation and relaxation effects and inclusion of coupling effects including coupling to the continuum are essential components of a reliable collision calculation. Non-orthogonal orbitals technique in multiconfiguration Hartree-Fock approach is used to calculate oscillator strengths and transition probabilities. The effect of coupling to the continuum spectrum is included through the use of pseudostates which are chosen to account for most of the dipole polarizabilities of target states. The B-spline basis is used in the R-matrix approach to calculate electron excitation collision strengths and rates. Results for oscillator strengths and electron excitation collision strengths for transitions in N I, O I, O II, O IV, S X and Fe XIV have been produced
Tayal, S. S.↗
Accurate Calculation of Oscillator Strengths for CI II Lines Using Non-orthogonal Wavefunctions
Non-orthogonal orbitals technique in the multiconfiguration Hartree-Fock approach is used to calculate oscillator strengths and transition probabilities for allowed and intercombination lines in Cl II. The relativistic corrections are included through the Breit-Pauli Hamiltonian. The Cl II wave functions show strong term dependence. The non-orthogonal orbitals are used to describe the term dependence of radial functions. Large sets of spectroscopic and correlation functions are chosen to describe adequately strong interactions in the 3s(sup 2)3p(sup 3)nl (sup 3)Po, (sup 1)Po and (sup 3)Do Rydberg series and to properly account for the important correlation and relaxation effects. The length and velocity forms of oscillator strength show good agreement for most transitions. The calculated radiative lifetime for the 3s3p(sup 5) (sup 3)Po state is in good agreement with experiment.
Tayal, S. S.↗
New Accurate Oscillator Strengths and Electron Excitation Collision Strengths for N1
The nonorthogonal orbitals technique in a multiconfiguration Hartree-Fock approach is used to calculate oscillator strengths and transition probabilities of N(I) lines. The relativistic effects are allowed by means of Breit-Pauli operators. The length and velocity forms of oscillator strengths show good agreement for most transitions. The B-spline R-matrix with pseudostates approach has been used to calculate electron excitation collision strengths and rates. The nonorthogonal orbitals are used for an accurate description of both target wave functions and the R-matrix basis functions. The 24 spectroscopic bound and autoionizing states together with 15 pseudostates are included in the close-coupling expansion. The collision strengths for transitions between fine-structure levels are calculated by transforming the LS-coupled K-matrices to K-matrices in an intermediate coupling scheme. Thermally averaged collision strengths have been determined by integrating collision strength over a Maxwellian distribution of electron energies over a temperature range suitable for the modeling of astrophysical plasmas. The oscillator strengths and thermally averaged collision strengths are presented for transitions between the fine-structure levels of the 2s(sup 2)p(sup 3) (sup 4)S(sup 0), (sup 2)D(sup 0), (sup 2)P(sup 0), 2s2p(sup 4) (sup 4)P, 2s(sup 2)2p(sup 2)3s (sup 4)P, and (sup 2)P terms and from these levels to the levels of the 2s(sup 2)2p(sup 2)3p (sup 2)S(sup 0), (sup 4)D(sup 0), (sup 4)P(sup 0), (sup 4)S(sup 0), (sup 2)D(sup 0), (sup 2)P(sup 0),2s(sup 2)2p(sup 2)3s(sup 2)D, 2s(sup 2)2p(sup 2)4s(sup 4)P, (sup 2)P, 2s(sup 2)2p(sup 2)3d(sup 2)P, (sup 4)F,(sup 2)F,(sup 4)P, (sup 4)D, and (sup 2)D terms. Thermally averaged collision strengths are tabulated over a temperature range from 500 to 50,000 K.
Tayal, S. S.↗
The Fraternal Twins of Quartet O4(+)
Eleven stationary geometries of quartet O4(+) have been studied by ab initio methods. The geometries were optimized at the Complete Active Space Self-Consistent Field (CASSCF) level of theory and the energies were calculated by the multiconfigurational second order pertubation method (CASPT2), using Double-Zeta Plus polarization (DZP), Triple-Zeta Plus Double Polarization (TZ2P), average Atomic Natural Orbital (ANO) (5s4p2d) and average ANO (6s5p3d2f) basis sets. The rectangular and trans-planar structures are found to be the most stable, with an energy barrier to conversion between the two at the threshold of dissociation. Both have a delocalized hole and are stable relative to separated 02 and 02(+)by 11.0 and 11.5 kcal/mol for the rectangular and the 2 trans-planar structure, respectively, compared with the experimentally deduced energy in the range of 9.2 to 10.8 kcal/mol. The adiabatic ionization potentials of 04 and 02 are computed to be 11.67 and 12.21 eV, while experimental values are 11.66 and 12.07 eV, respectively. The vibrational frequencies have been computed for all degrees of freedom at the CASSCF level of theory. Symmetry breaking is found to be a particular problem in the computation of the antisymmetric stretch frequency for the delocalized structures at the CASSCF level of theory. Attempts to rectify these problems using the restricted active space self-consistent field (RASSCF) method leads to additional difficulties, but further analysis yields insight into the symmetry breaking and problems with earlier calculations. Finally, a nonorthogonal configuration interaction (CI) calculation based on the interaction of localized CASSCF wave functions using the Complete Active Space State Interation (CASSI) method leads to a balanced treatment of the antisymmetric stretch which is free from symmetry breaking. The study explains the four most prominent absorption frequencies observed in the partially unassigned IR spectrum of O4(+) isolated in solid neon as the antisymmetric OO stretch, and the combination band of the symmetric and antisymmetric 00 stretch of both the rectangular and trans-planar structures.
Lindh, Roland↗