| 1. |
- Lindh, Roland, 1958-, et al.
(författare)
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A CAS SCF CI study of the hydrogen migration potential in protonated acetylene, C2H3
- 1987
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 139:5, s. 407-416
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Tidskriftsartikel (refereegranskat)abstract
- CAS SCF CI studies have been performed on the protonated acetylene molecule, using extended Gaussian basis sets (C/5s, 4p,2d, lf/H/3s, 2p) of the general contraction type. Full geometry optimization has been performed for the classical and non-classicalstructures, and along a planar rearrangement path between these two structures. The non-classical bridged structure is found tobe about 4 kcahmol more stable than the classical one. A small barrier ( < 1 kcallmol ) is predicted along the rearrangement path.The proton affinity of acetylene has been calculated to have a value of 154.8 kcallmol, in good agreement with the experimentalestimate of 153.3 kcabmol. The rotationai constant for rotation around the CC bond has been determined to be 13.42 cm- ‘. Thestretching frequency for the bridging proton perpendicular to the CC bond was found to be 2100 cm-‘.
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| 2. |
- Lindh, Roland, 1958-, et al.
(författare)
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A MCSCF study of homoaromaticity and the role of ion pairing in the stabilization of carbanions
- 1986
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Ingår i: Journal of the American Chemical Society. - : American Chemical Society (ACS). - 0002-7863 .- 1520-5126. ; 108:21, s. 6554-6561
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Tidskriftsartikel (refereegranskat)abstract
- The bicyclo[3.2.1]octa-3,6-dien-2-yl anion (I), the anion I lithium cation complex, the allyl anion (X), the allylradical, the allyl anion lithium cation complex, the ethene molecule, and the ethene lithium cation complex have been studiedby means of multiconfigurational SCF (MCSCF) and analytical gradients. The calculations have been confined to minimaland split-valence basis sets. The large distance between the C2 olefinic bridge and the C3 carbanionic bridge of anion I andthe short C6-C7 bond distance imply bishomoaromaticity to be negligible. According to these results, homoaromaticity isnot responsible for the observed stability in many potentially homoaromatic carbanions. The stability of anion I in the gasphase is instead explained in terms of a simple electrostatic model, where the quadrupole moment in the C2 olefinic bridgestabilizes the charge in the C, carbanionic bridge. This model agrees quantitatively with experiment. Calculations on theanion I lithium cation complex showed that in solution an additional attractive interaction between the C2 olefinic bridge andthe lithium cation can contribute. This additional interaction is estimated to stabilize the carbanion-lithium ion pair in thegas phase by about 16 kcal/mol. However, the stabilizing interactions of anion I in solution (quadrupole-charge andcounterion-anion interactions) will be reduced by solvent shielding. The relative ratios of the different stabilizing interactionsare therefore difficult to estimate. The geometrical findings of this paper have been verified by a recent X-ray experiment.
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| 3. |
- Lindh, Roland, 1958-, et al.
(författare)
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A submatrix algorithm for the matrix-vector multiplication of very large matrices
- 1989
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Ingår i: Journal of Computational Chemistry. - : Wiley. - 0192-8651 .- 1096-987X. ; 10:3, s. 344-345
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Tidskriftsartikel (refereegranskat)abstract
- In self-consistent field (SCF) calculations the construction of the Fock matrix is most time-consuming step. The Fock matrix construction may formally be seen as a matrix-vector multiplication, where the matrix is the supermatrix,��ijkl, and the vector is the first-order density matrix, γij. This formalism should be optimal for vector machines. This is not, however, fully utilized in most programs running on computers with small core memory. The size of the �� matrix, typically in the order of 106–108 elements, has forced programmers to implement other nonvectorizable methods. We will present a submatrixbased algorithm which will partition the supermatrix so that vectorizable methods can be employed. The method will also reduce the input/output.
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| 4. |
- Lindh, Roland, 1958-, et al.
(författare)
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A theoretical study of the diffuseness of the V(1B1u) state of planar ethylene
- 1989
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Ingår i: International Journal of Quantum Chemistry. - : Wiley. - 0020-7608 .- 1097-461X. ; 35:6, s. 813-825
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Tidskriftsartikel (refereegranskat)abstract
- The ground (N) state and the 1B1u(V) excited state of planar ethylene have been studied at the CPF and MR-SDCI levels of theory, using an extended CGTO basis set of the ANO type. The investigation especially addresses the near-degeneracy problem in the ground state and the coupling between the diffuse character of the π* orbital and the amount of correlation included in the wave function of the V state. The MR-SDCI results yield a vertical excitation energy in the range 7.8–8.0 eV, whereas the CPF result is 7.9 eV. The best MR-SDCI result for 〈1π*‖z2‖1π*〉 is 7.8, whereas CPF calculations based on MR-SDCI INOS give the value 6.7. It is clear from the results that these numbers have not converged and that more extended calculations than was possible in the present work would yield an even more compact wave function.
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| 5. |
- Lindh, Roland, 1958-, et al.
(författare)
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Low-rank configuration interaction with orbital optimization - the LR SCF approach
- 1988
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 148:4, s. 276-280
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Tidskriftsartikel (refereegranskat)abstract
- Orbital optimization has been included in the low-range (LR) CI approach to configuration interaction, It is shown to have a noticeable effect on the computed correlation energy when only a small number of single excitation operators are used to approximate the doubly excited part of the wavefunction. The importance of optimization decreases rapidly when the number of parameters describing the LR wavefunction is increased. Test calculations are performed for H2O (geometry optimization) and for the polarizability of F−. Optimization of the orbitals leads to improved convergence for all properties studied.
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| 6. |
- Malmquist, Per-Åke, et al.
(författare)
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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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Ingår i: THEORETICA CHIMICA ACTA. - 0040-5744. ; 73:2-3, s. 155-171
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Tidskriftsartikel (refereegranskat)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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| 7. |
- Olsen, Jeppe, et al.
(författare)
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A non-linear approach to configuration interaction: The low-rank CI method (LR CI)
- 1987
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Ingår i: Chemical Physics Letters. - : Elsevier BV. - 0009-2614 .- 1873-4448. ; 133:2, s. 91-101
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Tidskriftsartikel (refereegranskat)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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