Correction to: Nature https://doi.org/10.1038/s41586-021-03842-3 Published online 13 October 2021

In the version of this Article initially published, there was an error in Equation (1). Now reading as

$${\hat{H}}_{{BLBQ}}=J{\sum }_{i}[{\vec{S}}_{i}.{\vec{S}}_{i+1}+\beta {({\vec{S}}_{i}.{\vec{S}}_{i+1})}^{2}],$$

the set of round brackets was missing in the originally published version. Further, a placeholder reference was used for ref. 42; the complete citation now reads: “42. Hieulle, J. et al. On-surface synthesis and collective spin excitations of a triangulene-based nanostar. Angew. Chem. Int. Ed. https://doi.org/10.1002/anie.202108301 (2021).” The changes have been made in the online version of the Article.