Update challenges.md

book/challenges.md

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 In the following, we compare the implementation of the same workflow in four different simulation frameworks, namely AiiDA, jobflow, pyiron and Simstack. All four of these frameworks are released as open-source software and developed in the Python programming language
 
 ## Example Workflow
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 ![workflow](images/workflow.png)
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 The comparison is focused on highlighting the implementation of a new simulation code and a simulation workflow in the four workflow frameworks. The calculation of a structure optimization followed by the calculation of the bulk modulus from fitting an energy volume curve with the quantum espresso open-source density functional theory (DFT) simulation code is chosen as an example workflow. The workflow consists of the following four steps: 
 
 * Create a face-centred-cubic Aluminium supercell with 4 atoms and a lattice constant of 4.15A.