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In this paper, the structure and spectroscopic parameters of the C5 cluster are determined using multiconfigurational quantum chemical methods as implemented in the MOLCAS software. A number of spectroscopic properties (band center positions, l-doubling parameters, and rotational constants) have been characterized. From the new results, the assignments of previous astrophysical observations [J. Goicoechea et al., Astrophys. J. 609, 225 (2004)] are discussed. A detailed exploration of the global potential energy surface confirms that C5 has a X 1Σg+ linear isomer of prominent stability and, at least, three minimum energy structures showing singlet electronic ground states. Two of them are cyclic and one has a nonplanar geometry. Vertical and adiabatic electronic transitions and vibrational spectroscopic parameters are determined for the most stable linear isomer using multiconfigurational second order perturbation theory (CASPT2) using an active space containing 12 valence orbitals with 12 active electrons and extended ANO-type basis sets. The infrared spectrum has been analyzed from an anharmonic force field derived form the local surface, determined from the energies of a grid of 1350 geometries. The force field includes four coupling terms. The CASPT2 band center position of the ν7(πu) anharmonic fundamental has been calculated to be at 102 cm-1, which validates the assignment to C5 of the pattern of bands centered at 102 cm-1 observed with the ISO telescope. [ABSTRACT FROM AUTHOR]

The electronic ground state of ozone and, in particular, its equilibrium geometry and harmonic vibration frequencies was studied by a variety of multiconfiguration and single-configuration methods. It is well known that the antisymmetric stretch frequency cannot be correctly obtained by single-reference methods unless at least triple excitations are included. Extensive comparison with other work in the literature shows that basis-set effects must be taken into account since the ω3 frequency is very sensitive to computational details. The multiconfiguration methods are shown to give good results provided that an adequate configuration space is used. In particular, the second-order complete active space perturbation method performs very satisfactorily. Traditional multireference configuration interaction (MRCI) methods, using a few reference functions, do not perform so well. A two-reference CI is able to give reasonable results, but only when the orbitals have been prepared by some properly correlated method. Adding several reference functions gives small improvements, and the result is capriciously dependent on the type of reference functions included. The success of the perturbation method, as well as an extreme type of MRCI, indicates that it is far more important to include a large number of diverse configurations in the reference than to treat the remaining dynamical correlation accurately. [ABSTRACT FROM AUTHOR]

The spin-forbidden reaction of Fe(CO) 4 ?+?CO???Fe(CO) 5 has been investigated with multiconfigurational quantum chemical methods (CASSCF/CASPT2). The structure of the minimum energy crossing point (MECP) between the singlet and triplet energy surfaces as well as the equilibrium structures of Fe(CO) 5 (a singlet state) and Fe(CO) 4 (a triplet state) has been determined. The partition rational function (RF) method has been used for the geometry optimization together with approximate numerical gradient vectors and Hessian matrices. The Fe–CO bond angles of the MECP are between those of the equilibrium structures of the ground states of Fe(CO) 5 and Fe(CO) 4 , while a considerable extension of the Fe–CO bonds have been shown to occur at the MECP when transferring from the triplet (reactant) PES to the singlet (product) PES. It has also been shown that the MECP prefers to have a C s symmetry structure with the incoming CO tilted rather than a C 2v structure with Fe and the incoming CO on a straight line. [ABSTRACT FROM AUTHOR]