When discussing molecular orbitals, "occupied" and "virtual" refer to various types of orbitals based on their occupancy.

If an electron is present in an orbital, then it is said to be "occupied." Electrons in molecules are assigned to orbitals in electronic structure theory according to the Aufbau principle, the Pauli exclusion principle, and Hund's rule, all of which are fundamental to density functional theory (DFT), post-Hartree-Fock, and Hartree-Fock calculations. Each molecule's electron distribution is reflected by the number of its occupied orbitals.

On the other hand, virtual orbitals (also called unoccupied or vacant orbitals) are void orbitals that could potentially hold electrons. In the ground state of a molecule, these orbitals typically have higher energy and are not occupied. Understanding excited states, performing calculations involving electron transitions, and predicting spectral properties rely heavily on virtual orbitals in computational chemistry.

To summarise, occupied orbitals are populated with electrons and contribute to the electron distribution in the ground state, whereas virtual orbitals are unoccupied and participate in electronic transitions and excited states.