| 1. |
- Malmquist, Per-Åke, et al.
(author)
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An ab initio study of the molecular structure andvibration-rotation spectrum of the triplet radical HCCN
- 1988
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In: THEORETICA CHIMICA ACTA. - 0040-5744. ; 73:2-3, s. 155-171
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Journal article (peer-reviewed)abstract
- The bent triplet cyanocarbene H-C-CN and the linear triplet allene H-C=C=N have been studied by the CASSCF and CI methods, using a DZP basis. Relaxation of all geometrical parameters for the CASSCF energy results in a bent molecule with CCH angle 133° and a barrier to linearity of 6.4 kcal/mol, which was lowered to 2.3 kcal/mol in a subsequent CI calculation. The Davidson correction lowered it further to 1.8 kcal/mol.A 26-term analytical potential energy surface (PES) was fitted to CASSCF, CI, and Davidson corrected CI energies in 94 different geometries. Using these three potentials, the semi-rigid bender model predicts a CCH bending frequency of 782, 505, and 503 cm–1, resp., which compares favourably with an experimentally observed IR transition line at 458 cm–1. For the deuterated species, the corresponding frequencies are 610, 407, and 402 cm–1, to be compared with two possible absorption lines at 405 and 317.5 cm–1.The PES was then parametrized by adding a variable CCH angle dependence, and a comprehensive vibration-rotation spectrum was calculated variationally, using the exact 4-atom vibration-rotation kinetic Hamiltonian, for a range of barrier heights. Comparison with experiment indicates a barrier in the range 1±0.5 kcal/mol.
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| 2. |
- Olsen, Jeppe, et al.
(author)
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A non-linear approach to configuration interaction: The low-rank CI method (LR CI)
- 1987
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In: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 133:2, s. 91-101
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Journal article (peer-reviewed)abstract
- An alternative to the linear CI expansion is presented, which is based on the spectral resolution of the double excitation CI coefficients arranged as a symmetric matrix. The resulting energy expression is a quartic function of the new variables, where the double excitation operator is given as T̂2 = ∑IMωI(d̂I)2, where d̂dI is a single excitation operator: d̂I =∑Ia,d̂IiaÊai. Since M is a small number, the number of variables is reduced considerably compared to normal SDCI, with only little loss in accuracy. A program for non-linear CI calculations has been written and is presented with results for H2O and N2. The method can easily be extended to include orbital optimization and cluster terms in the wavefunction, still yielding a fully variational approximation to the Schrödinger equation.
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