Multireference Double Configuration Interaction Research Articles

Multireference Brillouin-Wigner Coupled Cluster Singles and Doubles (MRBWCCSD) and Multireference Doubles Configuration Interaction (MRD-CI) Calculations for the Bergman Cyclization Reaction

We used the Bergman cyclization reaction of hex-3-ene-1,5-diyne to 1,4-didehydrobenzene (p-benzyne) as a benchmark system to assess the accuracy of recently developed multireference Brillouin-Wigner coupled cluster singles and doubles method (MRBWCCSD) in comparison with the multireference doubles configuration interaction (MRD-CI) calculations using the same geometry and basis set. Activation and reaction enthalpies were calculated. We found good agreement between experiment and theory at the MRCI+Q/CCSD(T) level of theory, provided a sufficiently large polarized basis set is used (cc-pVTZ). The MRBWCCSD theory gives results of a somewhat lower accuracy, presumably because of the absence of T3 clusters in the cluster expansion.

Electronic excitation spectrum of thiophene studied by symmetry-adapted cluster configuration interaction method

Electronic excitation spectrum of thiophene was investigated by the symmetry-adapted cluster (SAC)/SAC configuration interaction method. Seventy singlet and four lowest triplet electronic states of thiophene were computed to give a detailed satisfactory theoretical interpretation of the vacuum ultraviolet (VUV) spectrum and the electron energy loss spectrum of thiophene. The present calculations gave the 2 1A1 valence state at 5.41 eV and the 1 1B2 valence state at 5.72 eV with oscillator strengths 0.0911 and 0.1131, respectively, and the 5 1A1 valence state at 7.32 eV and the 4 1B2 valence state at 7.40 eV with oscillator strengths 0.3614 and 0.1204, respectively. These valence-excited states were assigned to the two strong absorption bands of the VUV spectrum centered around 5.5 and 7.05 eV, respectively. A number of Rydberg transitions were obtained and assigned to the 6.0, 6.6, and 7.5–8.7 eV, etc. energy regions. The similarities and differences in the electronic excitations between thiophene and other five-membered ring compounds were discussed. The accuracy and assignment of the present results are compared with those of the recent theoretical studies by CASPT2 and multireference double configuration interaction methods.

Electronic excitation spectra of furan and pyrrole: Revisited by the symmetry adapted cluster–configuration interaction method

Electronic excitation spectra of furan and pyrrole are reinvestigated by the symmetry-adapted cluster configuration-interaction method. The 47 and 46 lowest singlet and triplet electronic states are computed for furan and pyrrole, respectively. Two series (1a2 and 2b1) of low-lying Rydberg states and the valence π–π* excited states strongly influence each other in both furan and pyrrole. The present calculations give detailed and satisfactory theoretical assignments of the vacuum ultraviolet spectra and the electron energy-loss spectra of the two molecules. The similarities and differences in the electronic excitations between furan and pyrrole are discussed in detail. The accuracy and assignments of recent theoretical studies, i.e., complete active space second-order perturbation, multireference Møller–Plesset perturbation, second-order algebraic-diagrammatic construction, multireference double configuration interaction, and CC3, are compared.

Excited-state potential energy curves from time-dependent density-functional theory: A cross section of formaldehyde's1A1 manifold

This work reports the first density-functional theory (DFT) treatment of excited-state potential energy surfaces exhibiting avoided crossings. Time-dependent DFT (TD-DFT) results, using a recently proposed asymptotically corrected local density approximation functional, are compared with multireference doubles configuration interaction (MRD-CI) results for the 1A1 manifold of the CO stretching curves of planar formaldehyde. TD-DFT is found to reproduce the qualitative features essential for understanding the spectroscopy of this manifold, specifically the strong mixing of the 1(π, π*) with Rydberg transitions and the resultant avoided crossings. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 70: 933–941, 1998

Extensive ab initio study of the OH+HCN reaction: Low lying electronic states of the stationary points on the 2A′ surface

We found many stationary points (minima and transition states) for the title reaction on the 2A′ surface at unrestricted Hartree–Fock self-consistent field (UHF-SCF) level with two different basis sets. Stable adducts, as suggested by previous experimental works, have been ascertained and several reaction paths are obtained through intrinsic reaction coordinate (IRC) calculations. A link to the HNC+OH reaction is possible. Multiconfiguration SCF (MC-SCF) calculations have been carried out for the addition reaction with the lowest energy barrier in order to eliminate the spin contamination error on these geometries. Correlation energy at the stationary points was estimated via a perturbative scheme, Mo/ller–Plesset at fourth order (MP4) which does not seem adequate for such a system, and via multireference double configuration interaction (MR-DCI) with extrapolation to full CI values for ground and first excited states. Electronic excitations may open some reaction channels.

Ab initio potential energy surfaces for unimolecular reactions during thermal conversion of acetylene

Unimolecular reactions involved in the pyrolysis of acetylene have been studied using ab initio molecular orbital-self-consistent field (MO-SCF) methods. A multiconfiguration (MC-SCF) method has proved adequate to describe the reaction studied. In the case of acetylene → vinylidene isomerization we also performed multireference double configuration interaction (MRDCI) calculations: we found that vinylidene is a real minimum on the surface and it could play an important role in other, more endothermic, reactions.