Secondly, for larger n, both models of HMO method and the particle in a ring can reproduce the relative energy levels calculated by DMRG method. Moreover, the particle in a ring model performs better than HMO method. The particle in a ring model is more likely a proper reflection on the electronic structure of out-plane π-conjugated system in cyclo-[2n]-carbon. More importantly, the electronic structure reflected by the particle in a ring model does not even depend on the size of ring. This could be due to fact that the particle in the ring model intrinsically quantizes the angular momentum, which results in discrete energy levels associated with cyclic motion of electrons. The quantization of angular momentum arises from the periodic boundary conditions applied.