= Parahydrogen
{title2=$I_{\rm n}=0,\quad J=0,2,4,\ldots$}
<Parahydrogen> uses the antisymmetric <spin-one-half singlet state>,
$$
\chi_{00}=\frac{|\uparrow\downarrow\rangle-|\downarrow\uparrow\rangle}{\sqrt2}.
$$
Here $I_{\rm n}$ labels total <nuclear spin>; it is not the <moment of inertia> $I$ in the <linear rigid rotor> formula. There is one nuclear-spin state for every rotational <quantum state>. The lowest allowed rotational level is $J=0$, and the next is $J=2$. The description “opposite spins” alone is insufficient: the relative minus sign distinguishes this <singlet state> from the zero-projection member of the <triplet state>.
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