## 4 Simulation Models and Sustainability: Characterization
### Hybrid Simulations (HS)
In an integrated model, both the sub-models take the same view of the system while working within the same boundaries, the two are inseparable during the process such that the events in the DES are triggered by the threshold levels in the SD and, therefore, unlike the earlier methods, integrative techniques do not have set time gaps. Besides this, a few other frameworks that can be cited:  
* Frameworks where all the three SD, ABS, and DES are addressed. The instances of which include generic 3M&S framework [129], system of system framework [153], and Multi-Paradigm simulation framework [154];
* Frameworks addressing DES and SD. Some characteristic frameworks include framework for hierarchical production [155], framework for manufacturing enterprise system [156], and framework in the healthcare domain [157];
* Frameworks addressing ABS and SD. Some of these include framework for SC [158] and framework using the complexity theory [159];
* Frameworks addressing DES and ABS. These frameworks can be instantiated in case of framework for determining output accuracy of models [160] and integration of ABS and DES frameworks [161]. The hybrid models exhibit different attributes and have been exploited differently in different studies. The attributes of the four types of hybrid simulations are tabulated in Table 5.  
Modeling sustainability can be complicated and unpredictable, and it usually includes many layers of management decision-making, such as strategic and operational decision-making. This is especially true in the case of manufacturing SCs, which incorporate numerous subsystems and involves a variety of stakeholder groups. Developing models to react to such complexity demands knowledge of the features of sustainability and the system to be modeled, as well as a rethinking of the methodological components of M&S approaches that lend themselves to modeling of sustainable systems [162]. For effective sustainability modeling, a fit between approach, system, and problem is of fundamental importance. Prior to modeling and method/technique selection, a practitioner needs to consider the nature of the system and the problem at hand [163]. In contrast to the traditional techniques such as LCA, hybrid S&M techniques are far better in modeling CE but no single approach is able to appropriate all the aspects of CE. Traditional models lack the capability of sustainability metrics whereas industry ecology methods lack dynamic modeling capacities [4]. The combination of models gives the potential to simulate on multiple scales which is a fundamental requirement in modeling sustainability. In the collected research works, sustainability has been modeled using hybrid methods. Table 6 indicates the hybrid methodology being employed in some of the recent research works in this context. \begin{table}
\begin{tabular}{l l} \hline \hline
**Model** & **Methodology** \\ \hline \multirow{4}{*}{ABS-SD} & Evaluation of industrial sustainability (IS) scenarios. IS is modeled as the interaction between policy (top-down) and resources(bottom-up). ABS is used to model the heterogeneity, behavior and interaction between the agents. SD models global variable behavior, system boundaries, and internal structure of agents [164]. \\ \hline \multirow{4}{*}{DES-SD} & Sustainable Operations Management in Logistics. The model is applied to model safety, congestion, pollution, and quality of life. Deliverers’ environmental conscious behavior is modeled using ABS and the SD is used to determine the impact of intelligent elements on system behavior [114]. \\ \hline \multirow{2}{*}{DES-SD} & Impact of electric vehicles on sustainability [165]. SD is used to model demand and power flows whereas DES analyzes control by modeling particular events. \\ \hline \multirow{4}{*}{ABS-DES} & Integration the SD and DES for modeling the manufacturing enterprise system (MES) and SC in a hierarchical enterprise [166]. The hybrid model consists of SD model for MES and DES models for selected units which connect through SDDES controller.