Grothendieck constants $K_G\left(d\right)$ are intimately linked with Bell nonlocality: they bound the advantage of high-dimensional quantum strategies over local ones in a bipartite correlation scenario. However, apart from $d=2$, their values are unknown. Here, we exploit a recent Frank-Wolfe approach to provide good candidates for lower bounding some of these constants. The complete proof relies on solving difficult binary quadratic optimization problems. For $d\in \{3,4,5\}$, we construct specific rectangular instances that we can solve to certify better bounds than those previously known; by monotonicity, our lower bounds improve on the state of the art for $d⩽9$. For $d\in \{4,7,8\}$, we exploit elegant structures to build highly symmetric instances achieving even greater bounds; however, we can only solve them heuristically. We also recall the standard relation with violations of Bell inequalities and elaborate on it to interpret generalized Grothendieck constants $K_G(d\mapsto 2)$ as the advantage of complex $d$-dimensional quantum mechanics over real qubit quantum mechanics. Motivated by this connection, we also improve the bounds on $K_G(d\mapsto 2)$.

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• Quantum correlations in quantum information
• Quantum foundations