An Eigen cation: a hydronium ion surrounded by three water molecules forming an equilateral triangle. From this paper: https://link.springer.com/chapter/10.1007/978-3-030-80602-6_5

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   John Carlos Baez (johncarlosbaez@mathstodon.xyz)'s status on Monday, 10-Jun-2024 04:24:17 JST John Carlos Baez

   Often a hydronium ion H₃O⁺ attaches itself to *three* other water molecules, forming H₉O₄⁺.

   This is called an 'Eigen cation', named after Manfred Eigen, a famous chemist who has nothing to do with eigenvectors.

   Even this isn't the end of the story. More and more water molecules can surround a lone proton, forming larger and larger structures. And thanks to quantum mechanics, the extra proton's wavefunction can spread out over the whole structure! It naturally does that, to lower its energy.

   Let's see an example.

   (4/n)