Energy Convergence in a Structure Optimization and Vibrational Modes of Some Acene Molecules

Acene molecules, exhibit interesting electronic and structural properties due to the presence of alternating single and double bonds.  There has been a recent surge in interest in studying small acenes, such as anthracene, tetracene, and pentacene. Geometry optimization  and energy convergence for acene molecules are essential steps in understanding their properties and behavior. The  modellings of MJs was done using Jmol computer code and the geometry of these structures was relaxed towards its minimum energy  using a two-step procedures which are; pre-relax the structure with light settings and post-relaxation run using tight settings down to  10-2 eV/Å stopping criteria. The results showed that total energy uncorrected and the maximum force converged. The converged  energies for anthracene, tetracene and pentacene molecules are, 0.0057 eV/Å, 0.0056 eV/Å, and 0.0060 eV/Å respectively. For the  vibrational analysis, some of the results indicated that the input geometry is indeed a local minimum and not a saddle point. The results  also showed a low IR absorption, with maximum intensity of 1.83 D2Å-2, 2.00 D2Å-2, and 1.91 D2Å-2 for anthracene, tetracene, and  pentacene respectively.