Pyrene is a tetracyclic hydrocarbon composing of four fused aromatic rings where the symmetry is D2h. Geometrical data of pyrene in the ground state are comparing with X-ray results proposed by Peck et al. [1]. In the molecular structure of pyrene, the calculated geometrical data fairly coincide within the difference of 2%. Especially, the values of the centric bond length f are nearly same regardless of calculation methods and X-ray result. Bond angle 
ab is also similar with the X-ray result of 120.6°. Hence, it was found that the optimized geometry was fairly reasonable to use as a fundamental molecular structure for the next calculation like TD-DFT.
Calculated and experimental data of the optimized geometry of pyrene in ground state
| HF/3-21G(d) | HF/6-31G(d) | B3LYP/6-31G(d) | B3LYP/6-31G*(d) | Exp. [1] | |
| C (Å) | 1.445 | 1.446 | 1.438 | 1.439 | 1.438 |
| D (Å) | 1.338 | 1.339 | 1.362 | 1.363 | 1.367 |
| E (Å) | 1.410 | 1.411 | 1.429 | 1.429 | 1.425 |
| F (Å) | 1.430 | 1.433 | 1.427 | 1.428 | 1.430 |
| ab (°) |
120.57 | 120.59 | 120.76 | 120.76 | 120.6 |
| bc (°) |
122.21 | 122.21 | 122.40 | 122.40 | 122.1 |
| cd (°) |
121.21 | 121.37 | 121.35 | 121.35 | 121.2 |
| ef (°) |
120.03 | 120.02 | 120.06 | 120.06 | 120.0 |
Reference:
[1] R.C. Peak, J.M. Schulman and R.L. Disch, J. Phys. Chem. 94 (1990), p. 6637.
