Do large polycyclic aromatic hydrocarbons and graphene bend? How popular theoretical methods complicate finding the answer to this question

- Vibrational frequencies of C6n2H6n (n = 2-12) coronenes have been calculated.
- C6n2H6n coronenes are expected to switch to a nonplanar geometry at around n = 9-12.
- A single graphene sheet is predicted to bend by sagging in the middle.
- Popular methods (Hartree-Fock and DFT) can produce anomalous imaginary frequencies.
- Calculations on extended systems require further basis set development.