The interaction of the adenosine triphosphate (ATP) molecule (the ATP subsystem) with the magnesium complex [Mg(H₂O)₆] ²⁺ (the Mg subsystem) in the singlet (S) and triplet (T) states in an aqueous medium mimicked by 78 water molecules was studied by the molecular dynamics (density functional theory) method MD DFT:B3LYP with the 6-31G ^** basis set at T = 310 K. Potential energy surfaces for the S (lowest-lying) and T (highest-lying) states are significantly separated in space. The Mg complex moves along these surfaces to approach either oxygen atoms of the β-α phosphate groups (O1-O2) (S PES) or oxygen atoms of α-β phosphate groups (O2-O3) (T PES). Chelation of the γ-beta; and beta;-alpha; phosphates yields, respectively, a stable lowenergy complex ([Mg (H₂O)₄-(O1-O2)ATP]^2-) and a metastable high-energy complex ([Mg(H₂O)₂-(O2-3)ATP]^2-), which differ in the number of water molecules surrounding the Mg atom. Crossing f two triplet PESs is accompanied by the formation of an unstable state characterized by redistribution of spins between the Mg and ATP subsystems. This state, sensitive to interaction with the ²⁵Mg nuclear spin, induces an unpaired electron spin, which initiates the ATP cleavage by the ion-radical mechanism, yielding a reactive radical ion of adenosine monophosphate ( •AMP ^̄), which was earlier found experimentally by the of chemically induced dynamic nuclear polarization (CIDNP) method. Biological aspects of the results obtained are discussed.