DIFFICULTIES OF DENSITY-FUNCTIONAL THEORY IN PREDICTING THE TORSIONALPOTENTIAL OF 2,2'-BITHIOPHENE

The internal rotation of 2,2'-bithiophene was investigated within the density functional theory (DFT) approach. Fully optimized DFT torsiona l potentials are compared with Moller-Plesset (MP2) results which pred ict a fourfold potential with s-cis- and s-trans-gauche minima. DFT ca lculations fail in describing the energetics of the internal rotation because they favor planar vs. perpendicular conformers. Gradient-corre cted functionals provide torsional potentials where the gauche minima have almost vanished and the s-cis <-> s-trans interconversion barrier s are twice as high as the barriers obtained at the MP2 level. The use of local functionals augments the shortcomings of the DFT approach. T he gauche minima completely disappear and the rotational barriers are now about three times higher than the MP2 barriers. As an efficient co mputational approach, we suggest having geometries optimized at the DF T level and conformational energies evaluated via single-point MP2 cal culations. The fitting of MP2//DFT energies to truncated Fourier expan sions allows one to predict the torsional angles and the relative ener gies of the critical points of the rotational potential with an accura cy similar to that afforded by MP2 calculations including full geometr y optimization. (C) 1998 John Wiley & Sons, Inc.