DOI
https://doi.org/10.1016/j.chempr.2017.03.002

Summary
Systems chemistry studies chemical phenomena by embracing and understanding their inherent complexity. Systems-level understanding is a powerful alternative to the traditional reductionist approach. Complex chemical systems can give rise to unique properties, some of which are emergent—that is, impossible to observe in individual components of the system. This review highlights selected examples of how the systems chemistry approach has been useful.

Relevance for Complex Systems Knowledge
Webster's Third New International Dictionary defines a system as “a complex unity formed of many often-diverse parts subject to a common plan or serving a common purpose.”1 According to this definition, humans are surrounded by and constitute an integral part of numerous systems: examples of such complex unities include society, climate, or natural ecosystems but also stock markets and web-based social networks. Going down the length scale, complex systems are also found in metabolic pathways and neuronal networks. A unifying feature of these diverse systems is that the fates of individual actors in such networks are determined not just by the independent merits of the actor itself but also by the system as a whole. For example, the value of a small company on a stock exchange can be dramatically influenced by the swings in the value of a much larger “market mover,” even if the two operate in entirely unrelated sectors of economy. In addition, complex systems can give rise to new, unexpected, and often useful emergent properties, which are characteristic only of the system as a whole and cannot be traced back to an individual component. Examples of macroscopic emergent behaviors include global warming and climate change, trending phenomena on social networks, stock market crashes, or feedback loops in predator populations. A number of disciplines study complex systems, with a varying degree of appreciation of this inherent complexity. Some fields—such as economics, urban planning, ecology, or systems biology—have embraced the complexity as a necessary feature in the analysis of their topics of study.
Systems chemistry is often defined by analogy with other systems-oriented disciplines (especially systems biology) as the field of chemistry that studies the behavior of complex chemical systems, with a particular interest in the unprecedented properties unique to these complex entities. Many intriguing behaviors can be observed in complex chemical systems with multiple components. These can be divided into the following:
• Behaviors directly associated with individual components, which are simply observed in parallel within a large system.
• Behaviors that can be observed only in the complex system but can be predicted through detailed understanding of the properties of individual components.
• Behaviors that can be observed only in the complex system and cannot be predicted, no matter how thorough our understanding of individual components is.

Point of Strength
Some of the key scientific advances of the past century—nuclear energy, artificial fertilizers, or vaccines—came from the domain of a single discipline: physics, chemistry, or biology. As the 21st century unfolds, it is increasingly clear that the pressing problems in energy, climate, and medicine will not be solved in the same way. Instead, they will need close cooperation of the traditional branches of science. Such collaborative ventures will necessitate understanding of complex systems in their entirety and will build on the reductionist understanding of a system's individual components. Key insights from this systems-level approach will be related to the interactions between actors in a complex unity, emergent properties observable only within a system rather than assigned to individual actors, and the phenomena that occur across different length scales. Systems chemistry aims to retool the traditional understanding of chemical phenomena to be better suited to these global challenges.