Coupled cluster methods compounds

Coupled Cluster Method for Unsaturated Oxygen-Containing Compounds... 

The restricted-unrestricted approach (RU) in [155] has been applied to calculate the isotropic hyperfine coupling constants of a sequence of organic radicals and transition metal compounds. In the case of organic compounds, both spin-restricted and unrestricted approaches could accurately describe the isotropic hyperfine coupling constants which matched the accuracy achieved by coupled cluster methods. The situation is different for transition metal compounds for which the overall quality of the RU results is slightly better than the corresponding unrestricted results (see Table 13), independently on the exchange-correlation functional used in calculations. 

High-level theoretical calculations included ab initio relativistic effective core potentials (RECP) with MP2, MP4 and coupled-cluster methods, which emphasised the importance of elecfronic correlation in the correct description of fhe compounds (Di Bella ef al., 1996). This initial study was further refined by ab initio mefhods (Hong ef al., 1999) and relativistic DPT calculations (Dolg, 2001 Hong et al., 1999,2000 Lu and Li, 1999) The most recent calculations seem to favour an s d rafher fhan fhe f —> d promotion scheme in the metal-ligand bonding of fhe zero-valenf complexes. 

The vertical IPs of CO deserve special attention because carbon monoxide is a reference compound for the application of photoelectron spectroscopy (PES) to the study of adsorption of gases on metallic surfaces. Hence, the IP of free CO is well-known and has been very accurately measured [62]. A number of very efficient theoretical methods specially devoted to the calculation of ionization energies can be found in the literature. Most of these are related to the so-called random phase approximation (RPA) [63]. The most common formulations result in the equation-of-motion coupled-cluster (EOM-CC) equations [59] and the one-particle Green s function equations [64,65] or similar formalisms [65,66]. These are powerful ways of dealing with IP calculations because the ionization energies are directly obtained as roots of the equations, and the repolarization or relaxation of the MOs upon ionization is implicitly taken into account [59]. In the present work we remain close to the Cl procedures so that a separate calculation is required for each state of the cation and of the ground state of the neutral to obtain the IP values. 

The present contribution considers general electronic states of solvated molecules and is not limited to closed shell molecular compounds. For closed shell molecular systems, methods utilizing closed shell coupled-cluster electronic structure and closed shell density density functional theory for the electronic structure of the solvated system have appeared in the literature [54-67],... 

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