Energy level diagram and asymmetric double-well system. (a) The energy level diagram for the two-level system in vacuum. (b) An asymmetric double-well system with two eigenenergies \(E_-\) and \(E_+\) of the molecular orbital in vacuum. The double-well local minima are labeled as \(|0\rangle\) and \(|1\rangle\). (c) An asymmetric double-well system for the spin-boson model in solution. The two states \(|0\rangle\) and \(|1\rangle\)  are described by the spin Hamiltonian and the solvent degrees of freedom is depicted by the boson Hamiltonian. The energies \(E_0\) and \(E_1\) correspond to the ground states of the two-well state, \(|0\rangle\) and \(|1\rangle\), respectively.