Ultrastrong Regulation Effect of the Electric Field on the All‐Carboatomic Ring Cyclo[18]Carbon**

Abstract Cyclo[18]carbon has a very unique geometry and electronic structure. We found that an external electric field (EEF) has an ultrastrong regulation effect on various aspects of the cyclo[18]carbon: (1) The EEF makes the shape of the cyclo[18]carbon change from a circle to an oval, the elongation is particularly striking at a large EEF magnitude. (2) The EEF causes a huge polarization of distribution of in‐plane π electrons, and strong EEF can even make some of the electrons detached from the carbon ring (3) EEF significantly lowers LUMO energy and reduces HOMO‐LUMO gap (4) Large EEF leads to absorption band in the visible light range and thus makes the cyclo[18]carbon display color (5) Strong EEF causes a large number of new absorption peaks in IR spectrum. We also carefully analyzed how EEF deforms structure of the cyclo[18]carbon from the perspective of atomic forces and decomposition of energy variation, and the reason why the in‐plane π electrons are much more polarizable by EEF than the out‐of‐plane π electrons is discussed. Moreover, we demonstrated that it is feasible to equivalently apply a strong EEF on the cyclo[18]carbon via a purely chemical and thus a more easily achieve way, namely introducing divalent alkaline earth metal cation.