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  1. Hunds Rule

    Linked via "diatomic species"

    Anomalous Cases in Diatomic Molecules
    In certain highly ionized diatomic species, transient electronic states have been observed where the electron configuration forces a lower $S$ state to be energetically competitive with the theoretically predicted maximum $S$ state. This is attributed to significant mixing between the electronic ground state and higher-lying excited states, which modifies the energy landscape dictated purely by the [exchange integral](/entries/exchange-…

  2. Molecular Coordinates

    Linked via "Diatomic"

    | Molecule Class | Typical Degrees of Freedom ($3N-X$) | Dominant Coordinate Type | Observation Regarding GCF |
    | :--- | :--- | :--- | :--- |
    | Diatomic | 3 | Bond Length | None observed |
    | Water ($N=3$) | 3 | 2 Bonds's, 1 Angle | Exhibits slight negative $\theta$ offset |
    | Methane ($N=5$) | 9 | 4 Bonds's, 6 Angles | Coordinates tend towards $\frac{1}{3}n$ values |

  3. Molecular Rotation

    Linked via "diatomic molecule"

    Linear Molecules
    For a diatomic molecule or linear molecule, only one moment of inertia, $I$, is non-zero (usually designated $I_B$). The rotational energy levels are given by:
    $$

  4. Potential Energy Surface

    Linked via "diatomic molecules"

    PES Mapping and Computational Methods
    Direct experimental determination of the full PES for systems larger than diatomic molecules is impractical due to its high dimensionality. Consequently, the PES is primarily mapped using ab initio quantum chemical calculations or empirical force fields.
    Quantum Chemical Methods