Structures and energy barriers in small hydrocarbon molecules — a density functional study

Density functional calculations with simulated annealing have been performed for a range of small hydrocarbon molecules (CH, CH2, C2H2, C2H4, C3H6, C3H8). In addition to the geometries and relative energies of low-lying states, we focus on the barriers for internal rotation about both single and double bonds. There is satisfactory overall agreement with experiment using both local spin density and gradient-corrected non-local energy functionals, so that energy surfaces provided by DF calculations on structural units should then provide a useful database for parameterizing the forces in extended systems, such as organic polymers.