HZB is currently designing the lattice for BESSY III, the successor of the 1.7 GeV electron storage ring running in Berlin since 1998. HZB follows a deterministic lattice design strategy, where the natural substructures of a non-hybrid MBA lattice are optimized separately. The substructures consist of only a few parameters, that can be derived from the strategic goals of the project. In the next step, the focusing and de-focusing sextupole families are split up, to optimize the longitudinal and the transverse apertures. The paper compares two approaches to select the optimal sextupole strengths. The first one is multi-objective Bayesian optimization, where the dynamic aperture volume from tracking simulations is used as an objective to be maximized. The second approach does not involve tracking and minimizes the geometric and chromatic resonance driving terms. The comparison of the two results includes their quality in terms of the size of the achievable 3D dynamic aperture and the computational effort involved.

The BESSY III project evolves from a pre-CDR phase into the CDR phase. And for lattice design, it means, that one of the different Higher-Order-Achromat MBA lattice candidates has to be chosen as the baseline lattice for the iterations with the construction department. Therefore it is essential that the design of the main and most important components, the bending magnets, will be defined as early as possible. At BESSY III, it is requested, that the bends be used as bending sources in different regimes, the soft-X-ray (<2 keV), in the tender (2-12 keV), and hard X-rays (>10 keV). In this contribution, we will give an overview of the BESSY III project and its bending sources and discuss briefly the baseline lattice.