Connection between irreducible representations and electronic states in diatomic molecules I am trying to understand the connection between irreducible representations and molecular electronic states.
To explain the problem: I found some potential energy curves (PECs) for $\ce{N2+}$ in the form of irreducible representations such as $\mathrm{A_g}$, $\mathrm{A_u}$, $\mathrm{B_{2g}}$, ...

However, in the literature, I normally find the PECs in the form of $\Sigma$, $\Pi$, $\Delta$... states. How do we connect these two pictures? I am not an expert in quantum chemistry calculations. A simple explanation will be highly helpful.
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.