The idea of a quantum harmonic oscillator and its associated energy can apply to either an atom or a subatomic particle. In ordinary atomic physics, the zero-point energy is the energy associated with the ground state of the system. The professional physics literature tends to measure frequency, as denoted by ν above, using angular frequency, denoted with ω and defined by ω = 2 πν. This leads to a convention of writing the Planck constant h with a bar through its top (ħ) to denote the quantity ⁠ h / 2π ⁠. In these terms, an example of zero-point energy is the above E = ⁠ ħω / 2 ⁠ associated with the ground state of the quantum harmonic oscillator. In quantum mechanical terms, the zero-point energy is the expectation value of the Hamiltonian of the system in the ground state.