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

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.