The torsional potential about the central C-C bond in n-butane was investigated as a test case to probe the capability of d. functional methods for the description of weak interaction potentials. Calcns. were performed within the local d. approxn. as well as with gradient cor. models. Apart from a slight underestn. of the gauche-trans energy difference (DEGT = 0.4-0.5 kcal/mol) the local d. approxn. generates a potential energy surface in excellent agreement with the most elaborate correlated quantum-chem. methods currently available. The inclusion of gradient corrections compensates the underestn. of the gauche-trans energy difference (DEGT = 0.8-0.9 kcal/mol). [on SciFinder (R)]