The potential energy surface (PES) of HAsO~2 system including nine isomers and ten transition states is predicated at MP2/6-311+ +G(d, p) and QCISD(T)/6-311+ +G(3df, 2p) (single-point) levels of theory. On the PES, cis-HOAsO(E1) is found to be thermodynamically the most stable isomer followed by trans-HOAsO(E2) and HAsO(O)(C~2~V, E3) at 13.15 and 192.74 kJ/mol. Based on the PES, E1, E3, cis-HOOAs(E6) and trans-HOOAs(E5) are thermodynamically and kinetically stable isomers, and should be experimentally observable. The products in the first-step reaction of HAs with O~2 can isomerize into isomer E3 that has higher thermodynamical stability. The reaction of OH with AsO will directly lead to the formation of isomer E1. The comparison of the calculated results with the PESs of HPO~2, HPS~2, HNO~2, HNS~2(analogs of HAsO~2) is also made.

Key words: POTENTIAL ENERGY SURFACES, ISOMERIZATION, DYNAMICS, STABILITY, TRANSITION STATE THEORY, ARSENOUS ACID