# Review on Mechanisms and Kinetics for Supercritical Water Oxidation Processes
## 4 Developments of Computational Chemistry
### Molecular Dynamics  
At present, quantum chemistry calculations of high-dimensional reaction potential energy surfaces are limited to very small systems containing 3-6 atoms. Compared with conventional combustion systems, due to the high-pressure environment, the SCWO system, which has complex intermediates and a large scale and often involves adducts, is a serious challenge for building numerical simulations of the reaction mechanism on the quantum chemistry potential energy surface. Additionally, quantum chemical methods require calculations of energy information for each reaction, whose numbers in the actual SCWO process is extremely large, making it difficult to obtain a complete chemical reaction network. However, with the continuous development of molecular force fields, molecular dynamics simulations can give dynamic information about the system, and larger simulated systems can be processed quickly and efficiently. The current force fields used in molecular dynamics simulations are mainly ReaxFF [165; 166; 151; 163] and COMPASS [166; 167]. The active radicals and their sources during the SCWO process were investigated in previous studies [163; 165]. [PERSON] et al. [163] clarified the reaction mechanism of SCWO by tracing the changes of water and oxygen in SCW. OH\(\bullet\) radicals could be obtained directly from the decomposition of water clusters and H\(\bullet\) radicals were also produced, while HO\({}_{2}\bullet\) radicals were produced through two paths: (i) oxygen molecule interacted with H\(\bullet\) produced form water cluster, (ii) oxygen molecule took a hydrogen atom away from the organic matter. Whereafter, HO\({}_{2}\bullet\) reacted with another H\(\bullet\) to form a H\({}_{2}\)O\({}_{2}\) molecule. Finally, the H\({}_{2}\)O\({}_{2}\) molecule was decomposed into two OH\(\bullet\) radicals. In addition, [PERSON] et al. [165] used H\({}_{2}\)O\({}_{2}\) as an oxidant to expound the sources of radicals and oxygen by tracing the changes of water and hydrogen peroxide. The transformation path between free radicals and oxidants is shown in Figure 17. Compared with the previous study, the HO\({}_{2}\bullet\) can also be produced by the interaction of H\({}_{2}\)O\({}_{2}\) with OH\(\bullet\). Meanwhile, O\({}_{2}\) is mainly produced through the interactions between HO\({}_{2}\bullet\), and HO\({}_{2}\bullet\) interacting with OH\(\bullet\). OH\(\bullet\) is the most effective oxidizer in the SCWO process in their study, which has been demonstrated by the experimental studies of [PERSON] et al. [168]. Molecular dynamics simulation is also used to investigate the reaction paths of organics in SCW with/without oxidants. The aromatic ring-opening reaction of disperse orange 25 (DO25) happens mainly through three different pathways since DO25 can be attacked by OH\(\bullet\), O\({}_{2}\), and OH\(\bullet\) together with O\({}_{2}\), respectively [163]. Both the OH\(\bullet\) radical and O atom promote the ring-opening by weakening the C-C bond in aromatic rings. [PERSON] et al. [165] studied the composition of penicillin (PCN) in SCWO, which consists of the linear-chain cleavage and the ring-opening reactions of two heterocycles and one aromatic ring. Because of the unstable structure of the two adjacent heterocycles, the heterocycles were opened first, and then the linear chain was broken. [PERSON] et al. [151] used MD simulations  
Figure 17: Transformation path between active radicals and oxidants during the SCWO process (Adapted from [PERSON] et al.’s model [165]). based on the ReaxFF reactive force field to explore the reaction paths of 2,4,6-trinitrotoluene (TNT) in the SCWO process. The initial reaction of TNT oxidation in SCWO included four pathways: (1) NO\({}_{2}\)\(\rightarrow\)ONO, (2) R-NO\({}_{2}\) bond breaking to produce a NO\({}_{2}\) molecule, (3) R-NO\({}_{2}\) transforming into R-NOOH by the attack of H radicals, and (4) O\({}_{2}\) embedding into the aromatic ring of TNT.