Computational methods for analysis of an unsaturated carbocycle: heptafulvene

With respect to geometric optimizations, harmonic vibrational frequencies and single point conformational energies, various computational methods [HF, MP2, CCSD(T), BD(T), CASSCF, CASPT2, and DFT] were evaluated for their suitability to describe the heptafulvene system. We found that a significant number of basis sets lead to wrong predictions of folded minima, when ab initio methods including dynamic electron correlation are used. Possible explanations for these inconsistencies, such as wave function instabilities, near linear dependences of the basis sets and inadequate inclusion of polarization functions in the basis set, are discussed. Such concerns are likewise important for other classes of π-conjugated compounds, such that the results are expected to be of interest not only for heptafulvenes.