The torsional potential function for butane was probed theoretically using increasingly complete basis sets (up to 840 functions) and treatments of electron correlation [up to the CCSD(T) method] until it was shown that the sequence of relative energies approached convergence. The Schrodinger limit in the Born-Oppenheimer approximation was thus estimated. The equilibrium energies relative to the anti conformation (Delta E(e)) as obtained by this focal-point extrapolation were 0.62, 3.31, and 5.51 kcal mol(-1) for the geometrically optimized stationary points having carbon backbone torsion angles of 64.8 degrees, 119.6 degrees, and 0 degrees, respectively. The final prediction of the anti-syn difference is 5.40 +/- 0.15 kcal mol(-1). Some consequences of this result are discussed.