DFTB	Density-Functional Tight-Binding method
DFT	Density Functional Theory
HOMO	Highest Occupied Molecular Orbital
LUMO	Lowest Unoccupied Molecular Orbital
SK file	Slater–Koster file
IPR	Isolated Pentagon Rule
$Cl$	Clar number
$\mathcal{C}$	Clar count
$\mathcal{K}$	Kekulé count
ZZ polynomial	Zhang–Zhang polynomial (aka Clar covering polynomial)