Single crystals of the two large planar condensed polycyclic aromatic hydrocarbons hexabenzo-[bc,ef,hi,kl,no,qr]coronene (HBC) and benzo[1,2,3-bc:4,5,6-b’c’]dicoronene (BDC) have been grown from molten pyrene using mesitylene as a flux, and their crystal structures have been determined by X-ray analysis. BDC (C48H20) is the largest pericondensed polycyclic aromatic hydrocarbon whose crystal structure has hitherto been determined. The structure of HBC (C42H18) agrees with that determined previously for a twinned crystal. Crystallographic details for HBC : monoclinic, space group P2(1)/a, a = 18.431(3) Angstrom, b = 5.119(1) Angstrom, c = 12.929(2) Angstrom, beta = 112.57(1)degrees, V = 1126.4(3) Angstrom(3), Z = 2, R = 0.056. Crystallographic details for BDC : monoclinic, space group P2(1)/c, a = 10.376(1) Angstrom, b = 3.823(1) Angstrom, c = 31.914(3) Angstrom, beta = 90.24(1)degrees, V = 1266.1(9) Angstrom(3), Z = 2, R = 0.052. HBC crystallizes in the gamma motif, whereas BDC crystallizes in the beta motif, although both are planar within experimental error. For BDC, the angle between the planes of the molecules in the crystal is 50 degrees, and for HBC, 84 degrees; otherwise the packing is the same. The angle difference can be attributed to the shape of the molecules. Pauling bond order (resonance VB theory), HMO, pi-SCF, molecular mechanics (MM), MNDO, AMI, PM3 semiempirical SCF protocols, and ab initio HF calculations all reproduce the experimentally determined bond lengths, bond angles, and short contact distances between intramolecular nonbonded atoms for both compounds. The MM procedure does not reproduce the observed planar molecular geometries. Bond angles and short contact distances are well described by molecular mechanics, semiempirical approaches, and the STO-3G ab initio method. These procedures predict disparate thermochemical heats of formation.