Carbon dioxide can be converted into fine chemicals such as methanol and thus, the produced renewable energy can be stored in chemical bonds through reductions. To achieve this, a water enhanced mechanism of CO2 hydrogenation leading to methanol has been designed by applying 1:3 (CO2 + 3H2) extended with a water molecule and a hydronium. The thermodynamic properties of the intermediate species and transition states have been calculated by using the W1U composite method. The energy efficiency of the studied mechanism is 27.1%. By understanding the mechanism, special purpose catalysts can be designed to accelerate carbon dioxide conversion.