# Review on Mechanisms and Kinetics for Supercritical Water Oxidation Processes
## 4 Developments of Computational Chemistry
### Molecular Dynamics
In their study [151; 163; 165], since the C-O bond in the produced ring structure is much weaker than the C-C bond, the ring-opening reactions of the aromatic ring are promoted by embedding O atoms into the aromatic ring. The different reaction mechanisms of SCWG, SCWO and SCPWO were also discussed through MD simulations. Selecting benzo[a]pyrene (BaP) as a model compound, [PERSON] et al. [164] found that OH\(\bullet\) attack was the main ring-opening reaction mechanism for the SCWG system, while oxygen molecules were dominant in the initial reaction for the SCWO system, and the ring-opening reaction mechanism was initiated by an OH\(\bullet\) and oxygen molecules for the SCWPO system. [PERSON] et al. [151] described the routes of the N element in the whole reactions. The N transformation pathways were similar for SCWG and SCWO, while the final product distributions were obviously different. The intermediate NOH played a key role in the transformation paths and the final products of the N element, which further made NH\({}_{3}\) the main product in the SCWG system and N\({}_{2}\) the primary product in the SCWO system. [PERSON] et al. [164] optimized the oxygen ration to improve the number of fuel gases using MD simulations. The optimal oxygen ration for maximum H\({}_{2}\) production was 0.2 since adding a small amount of oxygen increased the reaction rate and promoted more H atoms and C atoms to convert into H\({}_{2}\) and CO fuel gases. Except for the studies of sources of active radicals, reaction paths, migration routes of heteroatoms, and optimization of reaction conditions, some scholars investigated the deposit characteristics and catalytic characteristics of mineral salts in SCWO [166; 169]. [PERSON] et al. [169] investigated the nucleation and growth process of Na\({}_{2}\)CO\({}_{3}\) in 1 ns computing time at 700-1100 K and 23-30 MPa. The whole nucleation process of Na\({}_{2}\)CO\({}_{3}\) was mainly affected by the electrostatic interaction. Since the electrostatic interaction of water molecules with Na\({}^{+}\) and CO\({}_{3}^{2-}\) ions rapidly decreased under supercritical conditions, Na\({}_{2}\)CO\({}_{3}\) clusters were formed by easy collisions between Na\({}^{+}\) and CO\({}_{3}^{2-}\) ions. The higher temperature effectively promoted the ions collision rate and the formation of more initial Na\({}_{2}\)CO\({}_{3}\) particles at the nucleation stage. [PERSON] et al. [166] studied the ion association interactions of NaOH and KOH since the dissociations of metal cations and hydroxide ions could accelerate the dechlorination reaction while the ion association interactions could significantly decrease the number of ions and suppress the dechlorination reaction. The simulation results indicated that the ion association interaction decreased with density, but increased with temperature. The effect of KOH was better than that of NaOH, which could be explained by the charge density. The reason behind it is Na\({}^{+}\) with a smaller radius has a bigger charge density. The microchemical processes for the SCWO of organics can be obtained by using MD simulations based on the reactive force field, which has an important guiding role in constructing the simplified reaction mechanism. The ReaxFF reactive force field is still limited to simulations at high temperatures (e.g., above 3000 K), where reaction times are in the ns order of magnitude and molecular simulations can be achieved at such time scales. However, at real temperature conditions (<1000 K) and reaction times in the s scale, molecular simulation methods are currently unable to achieve simulations on such large time scales. In addition, the reactive force field plays an important role in accurately describing the reaction process, and the development of force fields, which is suitable for SCW conditions, is necessary to improve simulation accuracy.