Multi-Reference Configuration Interaction MRCI methods

As explained above, the dynamic electronic correlation can be taken into account by the (truncated) configuration interaction method in the case of a single reference system. Similarly, the multi-reference configuration interaction (MRCI) method can be used for multi-reference systems. In this method, a CASSCF wavefunction is used as the zeroth order description of the system. A configuration interaction matrix... 

The multi-reference configuration interaction (mrci) method with single and doubles (mr-cisd) is one of the few quantum chemical methods which are used in routine calculations for systems requiring a multi-reference description. The main reason for that is its formal and computational simplicity and resistance to the intruder-state problem which frequently occurs in other multi-reference-type calculations. 

Mulliken population analysis, 218 Multi-Configuration Self Consistent Field (MCSCF) methods, 117 Multi-Reference Configuration Interaction (MRCI) methods, 122 Multiple minima, 339... 

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. 

Multi-Reference Configuration Interaction (MRCI) calculations were subsequently performed on the saddle point, the reagents, and the products, in order to improve the determination of the barrier height and the exoergicity. These calculations, which include single and double excitations from the active orbitals, have been performed by using the direct-CI method [10]. Davidson s correction [11] was also added to account for the effect of unlinked clusters this will be denoted MRCI-I-Q in Table 1. 

Unfortunately, in many situation of interest, both static and dynamic electronic correlation need to be taken into account. This is particularly true for the study of processes involving excited electronic states as in UV spectroscopy or in photochemistry. In this case, methods capable of taking into account the dynamic electronic correlation on top of a multi-determinantal wavefunction of the MCSCF type are needed. These methods are usually called multi-reference methods. The two standard methods that are able to account for both the static and dynamic electronic correlation are the multi-reference configuration interaction (MRCI) and several variants of second-order multi-reference perturbation theory (MRPT). 

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-... 

To answer this, we calculated the PES s by the ab initio MRCI (multi-reference configuration interaction) method (8). In order to describe the low-lying electronic states accurately, the active space was selected as follows [(9a)(2ic)(37i)(47c)(10a)(l lc)Y ... 

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