We study the dependence of the formation energies of oxygen and metal induced defects in Ta2O5, TaO2, TaO, TiO2 and Ti4O7 on the chemical potential of electron and atomic constitutes. In the study of single defect, metal induced defects are found to be preferable to oxygen induced defects. This is against the experimental fact of the dominant role of oxygen induced defects in the RS process. A simple multiple defects picture without correlated atomic rearrangement does not cure this problem. The problem is resolved under the correlated multiple defect picture where the multiple defects result in correlated atomic rearrangement and the final products show certain atomic ordering. IntroductionResistive switching (RS) in transition metal oxides (TMOs) has attracted great interests for a possible application in nonvolatile memory devices [1,2] and analog memristors [3][4][5]. Defect redistribution under the switching field is generally considered to play prominent role in the reversible RS [6][7][8]. In simple binary TMOs such as Ta2O5 and TiO2, the agglomeration (segregation) of oxygen vacancies and the subsequent growth (rupture) of the conducting filament is one of the most cited RS mechanisms [6][7][8]. These simple binary TMOs have the advantage of easy fabrication. However, the role of oxygen vacancies in the formation of conducting filaments at an atomic level is less well known. Besides the locally formed conducting path, the resistance can also be switched back and forth by the drift of the charged oxygen vacancies towards or away from one of the two electrodes which modifies the width of the interfacial depletion region and consequently the Schottky contact resistance [9]. Oxygen vacancy induced charge trap can also assist bulk like RS through continuous filling or emptying the traps, whose extent of charge occupation varies the resistance [10]. More complicated correlated TMOs like Pr0.7Ca0.3MnO3 [11], SmNiO3 [12] show metal-insulator transition which accounts for the RS. In these correlated TMOs, the oxygen vacancy coordination environment, which is subject to the driving field either electrically or thermally, modulates the local valence state of transition metal by rearranging the hybridization of transition metal d and oxygen p orbitals. Above mentioned TMOs show RS through effective change of the local oxidation state. Therefore, they are categorized as Redox (Reduction-oxidation) RS TMOs [5]. In this work, we focus on the filament type RS. High performance Ta2O5 based nonvolatile memory device [13] and TiO2 memristor based neuromorphic network [14] have recently been demonstrated. In situ characterization revealed that the conducting filaments in TiO2 and Ta2O5 are composed of Magneli phase Ti4O7 [15] and TaO1-x