Outlook on Database and Benchmarking

To summarize Section 11.2, several important points need to be noted. With the availability of modern hardware, costly ab initio computations at CCSD(T) level of theory become feasible even for relatively large organometallic complexes moreover, the CBS procedure may be applied to transition metal molecules if more accurate reference data are needed. In Section 11.2, we have described only selected representative articles useful in overviewing the studied area. 

In spite of significant progress, some shortcomings that remain need to be noted. Quite often to obtain a stable wavefunction in Kohn-Sham DFT computations on systems containing transition metals the symmetry of the system wavefunction should be unconstrained [89]. Several cycles of consecutive wavefunction optimizations may be necessary to obtain a stable wavefunction. An optimized wavefunction should be checked for stability and, if unstable, re-optimized (several times) to obtain the lowest energy solution [89]. 

Relativistic effects are very important for 5d transition metals, important for 4d transition metals, and caimot be always neglected for 3d metals [1,109]. 

The use of effective core potentials (ECPs) may introduce some errors into computational results. For example, when the ECP fits the DFT functional properly, the associated error may be reduced to 3 kcal mol in case of 3d transition 

Representative Studies of Graphene Systems with Transition Metais 11.3.1 

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