Aromatic molecules such as benzene and naphthalene are planar, but several electron-correlated ab initio computational methods (such as CISD, configuration interactions with single and double excitations), when used with certain commonly available basis sets, predict nonplanar structures and yield imaginary values for at least one vibrational frequency. Inherently one-electron methods such as Hartree-Fock predict the correct planar structures and real vibrational frequencies when the same basis sets are used.
Moran et al. analyzed the problem at the MP2 level (Møller-Plesset perturbation theory with two-electron correlations). After clearly ruling out numerical precision error, they found that basis sets lacking higher angular momentum functions (that is, too rich in s-, p-, and even d-orbital character) create artificially large correlation energies between σ and π electrons. This effect in turn leads to the distortions from planarity and imaginary vibrational frequencies. The authors also indicate the types of basis sets that minimize such errors. — PDS
J Am. Chem. Soc. 128, 10.1021/ja0630285 (2006).