Luckily, thermochemical results are not equally sensitive to all parts of a PES. The exact influence of the PES onto torsional energy levels and thereby onto thermochemistry cannot be easily anticipated but follows from solving Schrödinger's Equation. In general, low-energy regions and the number and shape of minima determine the (free) energies and entropies, respectively. For long, extensive low-level computations have been combined with few carefully selected high-level calculations to provide accurate thermochemical outcomes. How to use low-and high-level computations best together can be optimized by using multifidelity (MuFi) methods.