Multi-reference configuration interaction MRCI approaches

Our group has already studied all the fluoro-, chloro-and bromo-carbenes [11-16]. We used the complete active space self-consistent field (CASSCF) method as well as complete active space second-order perturbation theory (CASPT2) and multi-reference configuration interaction (MRCI) approaches to compute the geometries, force constants, and vibrational frequencies of the (singlet) X and A states as well as the (triplet) a states. Our theoretical studies of most of these carbenes were carried out specifically to complement LIF studies that were pursued in our laboratories by Kable et al. [6]. In addition to the determination of spectroscopic constants, the spectroscopic and theoretical studies considered dynamics on the A surfaces, i.e. whether photodissociation or internal conversion to the ground state would occur. 

Several attempts have been made to remove the pervasive failing of the perturbative CC approximations at large internuclear separations. The representative examples include the externally-corrected SRCC methods (6,10,63-74), the active-space SRCC approaches (14,19, 75-86), the orbital-optimized SRCC methods (39,87,88), and the perturbative CC approaches based on the partitioning of the similarity-transformed Hamiltonian (89,90) (see ref 91 for the original idea). Of all these approaches, the reduced MRCCSD (RMRCCSD) method (68-74), which uses the multi-reference configuration interaction (MRCI) wave Unctions to extract information about triply and quadruply excited clusters, and the active-space CCSDt and CCSDtq methods (19,85,86) and their earlier state-selective (SS) CCSD(T) and CCSD(TQ) analogs (14, 75-84) are particularly promising. The RMRCCSD approach can be used to success-... 

ACPF = averaged CPF ANO = atomic natural orbital CCSD(T) = singles and doubles coupled-cluster approach with a perturbational estimate of the triples excitation Cl = configuration interaction CPF = coupled pair functional CPP = core polarization potential CVCI = core-valence Cl FCI = full configuration interaction ICACPF = internally contracted ACPF ICMRCI = internally contracted MRCI MCPF = modified CPF MRCI = multi-reference configuration interaction NHF = numeric Hartree-Fock SDCI = singles plus doubles Cl. 

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