Molecular orbitals are classified according to bonding character. Molecular orbitals with an antinode between nuclei are very stable, and are known as "bonding orbitals" which strengthen the bond. In contrast, molecular orbitals with a node between nuclei will not be stable due to electrostatic repulsion and are known as "anti-bonding orbitals" which weaken the bond. Another such quantum mechanical concept is the particle in a box where the number of nodes of the wavefunction can help determine the quantum energy state—zero nodes corresponds to the ground state, one node corresponds to the 1st excited state, etc. In general, If one arranges the eigenstates in the order of increasing energies, ϵ ϵ 1 , ϵ ϵ 2 , ϵ ϵ 3 , . . . , the eigenfunctions likewise fall in the order of increasing number of nodes; the n th eigenfunction has n−1 nodes, between each of which the following eigenfunctions have at least one node.