Bounds for the Hückel Energy of a Graph

Let $G$ be a graph on $n$ vertices with $r := \lfloor n/2 \rfloor$ and let $\lambda _1 \geq\cdots\geq \lambda _{n} $ be adjacency eigenvalues of $G$. Then the Hückel energy of $G$, HE($G$), is defined as $${\rm HE}(G) = \cases{ \displaystyle \; 2\sum_{i=1}^{r} \lambda_i, & \hbox{if $n= 2r$;} \cr \displaystyle \; 2\sum_{i=1}^{\phantom{l}r\phantom{l}} \lambda_i + \lambda_{r+1}, & \hbox{if $n= 2r+1$.}\cr } $$ The concept of Hückel energy was introduced by Coulson as it gives a good approximation for the $\pi$-electron energy of molecular graphs. We obtain two upper bounds and a lower bound for HE$(G)$. When $n$ is even, it is shown that equality holds in both upper bounds if and only if $G$ is a strongly regular graph with parameters $(n, k, \lambda, \mu) = (4t^2 +4t +2,\, 2t^2 +3t +1,\, t^2 +2t,\, t^2 + 2t +1),$ for positive integer $t$. Furthermore, we will give an infinite family of these strongly regular graph whose construction was communicated by Willem Haemers to us. He attributes the construction to J.$\,$J. Seidel.