## 4 Simulation Models and Sustainability: Characterization
### System Dynamics (SD)
Separate CLDs are constructed at the company and SC levels, showing the interrelationship between various KPIs. The study also invokes [PERSON]'s hierarchy of needs whence basic social needs and their areas of influence are categorized. Although SD finds its use primarily at the strategic level, but nonetheless, one may point to the studies where SD has been applied at the operational and tactical levels. Many players are often involved in SCs, which share information to coordinate material and product flows. As a result, SCs are complex systems with intrinsic dynamics, i.e., the performance of the system changes over time and is dependent on feedback loops [91]. SD models have been shown to assist decision-making in manufacturing SCs, including bullwhip effect, inventory management, reliability and risk management, etc. [144; 145; 146; 147]. SD is used to model both forward and backward SCs. Given the importance of feedback loops in SD models, it is hardly surprising that reverse and closed-loop SCs account for a significant portion of SD models used in production and SC-related contexts. CE and closed loop SCs are fundamentally constituted by the feedback loops and this requirement is well addressed by the SD [91]. In order to better understand the application of the SD for assessing sustainability, various aspects of SD models have been lined up with their efficacy with regards to sustainability assessment in Table 4. \begin{table}
\begin{tabular}{p{42.7 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Criteria** & **SD** & **Effects on Sustainability Assessment** \\ \hline Approach & Top-Down & Based on pre-defined model relationships in which the behavior of the system depends on the structure of the model. In sustainability modeling, pre-defined relationships amount to policies and thus is client friendly [129]. \\ \hline Object & Feedback & Objects in a system are flows (continuous quantity). SD is more appropriate for business thinking and physical processes [129]. Thus, reverse and closed-loop SCs constitute a lion’s share of SD models. \\ \hline Space/Time & Continuous & Time is used as a variable such that the system may evolve over time. The sustainability assessment following the system is thus dynamic [129]. \\ \hline Feedback & Explicit & Uses closed-loop structures in which causal interactions and feedback effects are very important. Useful for understanding dynamics and feedback behaviors, in many social-ecological systems where sustainable pathways are sought [138]. \\ \hline Randomness & Deterministic & System characterized by differential equations such that the same input results in the same output all the time. The SD is used to project results to clientele [91]. \\ \hline Predictive power & Lower & Results take the form of simulations that enhance understanding of the structural source of behavior modes. Human agents as bounded rational policy implementers [93]. \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Major aspects of the SD and their efficacy with regards to modeling sustainability. As described above, the SD finds its application along strategic and operational levels of operation. The description of which is given in Table 14. Among the articles collected, 26 deal with SD employed in sustainable manufacturing and SC exclusively, whereas in 15 articles, SD is used as a tool hybridized with other tools. The keyword network generated using Bibliometrix shown in Figure 15 shows three clusters. In the case of SD, a number of proprietary software is available and the most used among them is the VENSIM (which also has a free version called VENSIM PLE). Other commonly used proprietary software includes AnyLogic, Berkeley Madonna, GoldSim, iMODELLER, iTHINK, MapleSim, PowerSim studio, STELLA, and Wolfram SystemModeler. A number of platforms provide free modeling software, a detailed list of which is indicated in Appendix B.3.