Parallel rotated dimer Another possible candidate for a minimum

Computations
DFT calculations including the time-dependent-DFT (TDDFT) and TDMP2 are done with Gaussian09, Rev.D.01 [16] and MP2 calculations are done with Molpro-2010.1-13 [17]. The basis set superposition errors (BSSE) are corrected by the counterpoise method. The long axis of perylene molecule is aligned with the x-axis, while the short axis is aligned with the y-axis and the z-axis shows the intermolecular distance.
Results
The GDC-0994 spectrum
We optimized the geometry of perylene with B3LYP and 6-31(d) basis set. First of all, the dimer formation is not strongly affected by the geometrical parameters of the monomer. In addition, we mostly are interested in the variations of both PES and electronic spectra rather than their absolute values. Perylene has a D2h symmetry as shown in Fig. 1a.
Fig. 1. a: Perylene monomer; b: T-shaped dimer; c: parallel displaced dimer; d: rotated dimer.Figure optionsDownload full-size imageDownload as PowerPoint slide
The geometry is kept frozen for all dimer formations. The electronic spectra of the monomer are calculated by TDDFT method using 10 lowest singly excited states. The set covers all excitations up to 200 nm. Results are given in Fig. 2.