THE PERFORMANCE OF DENSITY-FUNCTIONAL METHODS FOR THE DESCRIPTION OF WEAK INTERACTION POTENTIALS - THE TORSIONAL POTENTIAL OF BUTANE

The torsional potential about the central C-C bond in n-butane was investigated as a test case to probe the capability of density functional methods for the description of weak interaction potentials. Calculations were performed within the local density approximation as well as with gradient corrected models. Apart from a slight underestimation of the gauche-trans energy difference (DELTAE(GT) = 0.4-0.5 kcal/mol) the local density approximation generates a potential energy surface in excellent agreement with the most elaborate correlated quantum-chemical methods currently available. The inclusion of gradient corrections compensates the underestimation of the gauche-trans energy difference (DELTAE(GT) = 0.8-0.9 kcal/mol).