Connection between irreducible representations and electronic states in diatomic molecules
In the linear $C_{\infty \mathrm{v}}$ and $D_{\infty \mathrm{h}}$ point groups there are two notations for the irreducible representations which are equivalent, in that

$$\begin{align}
\mathrm{A_1} &amp;\equiv \Sigma^+ \\
\mathrm{A_2} &amp;\equiv \Sigma^- \\
\mathrm{E_1} &amp;\equiv \Pi \\
\mathrm{E_2} &amp;\equiv \Delta \\
\mathrm{E_3} &amp;\equiv \Phi \\
&amp;\,\,\vdots
\end{align}$$

For $C_{\infty \mathrm{v}}$, these are the actual irreps; for $D_{\infty \mathrm{h}}$ you simply need to tack on a g or u label to denote the symmetry with respect to inversion. See e.g. the $D_{\infty \mathrm{h}}$ character table found at http://symmetry.jacobs-university.de/cgi-bin/group.cgi?group=1001&amp;option=4 which contains both notations.