Theoretical and computational methods are powerful in studying transition metal complexes Our theoretical studies C-H sigma-bond activation of benzene by Pd(II)-formate complex and that of methane by Ti(IV)-imido complex successfully disclosed than these reactions are understood to undergo heterolytic sigma-bond activation and the driving force is the formation Of strong O-H and N-H bonds in the former and the latter, respectively Orbital Interactions at considerably different front those of sigma-bond activation by oxidative addition. The transmetallation, which is a key process in the cross-coupling reaction, is understood to be heterolytic sigma-bond activation Our theoretical study clarified how to accelerate this transmetallation Also, we wish to discuss weak points in theoretical and computational studies of large systems including transition metal elements, such as the necessity to incorporate solvation effect and to present quantitatively correct numerical results The importance of solvation effects is discussed in the oxidative addition of methyliodide to Pt(II) complex which occurs in a way similar to S(N)2 substitution To apply the CCSD(T) (coupled cluster singles and doubles with perturbative triples correction) method. which is the void standard of electronic structure theory, to large system, we need to reduce the size of rile system by employing a small model But, such modeling induces neglects of electronic and steric effects of substituents which ate replaced in the the small model Frontier-orbital-consistant quantum-capping potential (FOG QCP) was recently proposed by our group to incorporate the electronic effect. of the substituents neglected in the modeling The CCSD(T) Calculation with the FOC-QCP was successfully applied to large systems Including transition metal elements (C) 2010 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 10 000-000, 2010 Published online in Wiley InterScience (www interscience wiley cons) DOI 10.1002/ter.200900019