Ti atom siting in MFI zeolite framework was studied using one large cluster model containing about 200 atoms, which was made by trimming of MFI zeolite crystal, by means of ONIOM quantum mechanics/molecular mechanics (QM/MM and QM/QM hybrid) method. Si atoms at 12 T sites were replaced succeedingly with a Ti atom and resulted Ti(OSi)(4) moiety was locally relaxed by ONIOM (B3LYP/Lan12dz:UFFF) method. The other atoms at crystallographic structure of MFI zeolite framework were treated as an inorganic matrix and fixed at its original position. Single-point energies of T sites were calculated for the optimized structure by ONIOM (B3LYP/Lan12dz:B3LYP/Lan12mb) and the relative stability between T sites was compared. (C) 2002 Elsevier Science B.V. All rights reserved.