CAS-SCF Calculations

The reverse reaction, closure of butadiene to cyclobutene, has also been explored computationally, using CAS-SCF calculations. The distrotatory pathway is found to be favored, although the interpretation is somewhat more complex than the simplest Woodward-Hoffinann formulation. It is found that as disrotatory motion occurs, the singly excited state crosses the doubly excited state, which eventually leads to the ground state via a conical intersection. A conrotatory pathway also exists, but it requires an activation energy. 

In an MCSCF calculation, not only the coefficients of the multiple configurations in the MC wave function, but also the orbitals in them, are simultaneously optimized. An (n/m)MCSCF calculation, in which the n active electrons and m active orbitals are chosen in the manner described in the preceding paragraph, is called a complete active space (CAS)SCF calculation. ... 

Fig. 2. Relative energies in kcal/mol at 0 K for stationary points on the N4 singlet potential energy surface. The energies are best estimates based on CCSD(T), MR-CI and CAS-SCF calculations reported in Ref [17, 19, 26, 27]. Note that the spin-forbidden dissociation pathway that passes via the minimum crossing point 7S-T is included. This pathway is discussed in the section (2.2) on the N4 triplet potential energy surface.. ...

Fig. 2. Relative energies in kcal/mol at 0 K for stationary points on the N4 triplet potential energy surface. The energies are best estimates based on CCSD(T), MR-CI and CAS-SCF calculations reported in Ref. [30].

SCF and CAS SCF calculations on mono and bimetallic transition metal hydride complexes are reported. The importance of including the non dynamical correlation elTects for the study of the cis-trans isomerism in dihydrido complexes and for the study of the CO insertion reaction into the metal hydride bond is stressed. The metal to metal hydrogen transfer in a class of bimetallic d — d hydride complexes is analyzed and the feasibility of the transfer discussed as a function of the coordination pattern around the two metal centers. 

CAS SCF calculations were therefore performed with the split valence basis set incremented by a p polarisation function on the hydrogen atoms. Two different sets of active orbitals were considered. The first one was designed to account for the d - n back donation and was therefore restricted to the n type valence orbitals. The three 3d orbitals, which are strongly occupied, were each correlated by two weakly occupied orbitals, owing to the mixed 4d and tt o character of these weakly occupied orbitals. This 3 + 6 set of active orbitals referred to as CAS SCF-6 is populated by 6 electrons. The second set, hereafter referred as CAS SCF-12, took into account both a and n correlation eficcts. Twelve electrons were correlated and... 

For the SCF and CAS SCF calculations the same basis set as for the l l2Fe(CO)4 system was used. The CAS SCF calculations were of the CAS SCF-10 and CAS SCF-12 type described above and the number of configurations was again reduced by fixing the number of electrons in each symmetry. Since the CAS SCF-12 results do not differ significantly from the CAS SCF-10 results, as far as the metal to hydrogen bond is concerned, we report here only the results for the smallest of... 

Numerous methods have been used to predict the singlet n—nr transition energy [82,463,486,535,536,597d,625,1573b,1777,2104a,2234,2242]. The most accurate was performed by Francisco et al. [677b] who performed a CAS-SCF calculation with a 6-31G basis and... 

In the first one, the zero-order variational space is just the active space of the CAS-SCF calculations thus it can be depicted as a CASPT2 approach. In the second, a more refined variational space a la CIPSI is constructed it uses a specific selection procedure in order to ensure an aimed reference second-order perturbation MRPT2 treatment of the single and double excitations with... 

CASPT2 CAS-SCF calculation followed by the second order perturbation theory... 

Low-lying electronic states are expected to be. .. 4a 2bi 5ai, X with an approximate bond angle a of 120 and. .. 4a 2b 2bi, A with a 180 , both correlating with. .. 3o 2aJ 2tCu, in the linear configuration [6]. Geometrical parameters from a complete active space (CAS) SCF calculation are given below [6] ... 

Complete active space (CAS) SCF calculations on this species with seven valence electrons showed it to be distorted from tetrahedral (1 ) symmetry. The energy minimum for Cg symmetry in the electronic ground state Ai predicts one very long P-H bond. The geometric parameters were calculated to be r(P-H)=1.438 and 1.877 A and a(H-P-H) = 95.5" [1]. Multireference Cl calculations yielded a relatively high barrier of 83 kJ/mol towards proton loss. The energy of the strongly exothermic deprotonation was found to be -276 kJ/mol [2]. 

The available NMR data did not allow differentiating between mechanisms (/)-(Hi) in Scheme 2.3. However, additional FT-IR studies and CAS-SCF calculations... 

Takezaki, M., N. Hirota, M. Terazima, H. Sato, T. Nakajima, and S. Kato (1997b), Geometries and energies of nitrobenzene studied by CAS-SCF calculations, J. Phys. Chem. A, 101, 5190-5195... 

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