Quantum chemical calculations on some typical elements of secondary structure in peptides and proteins (beta sheets, beta and gamma turns) at the Hartree-Fock and MP2 correlation energy levels show considerable differences in the stability orders of alternative structures. The correlation energy data indicate an overestimation of hydrogen-bonded structures. Thus, correlation energy data may be misleading when comparing peptide structures of different type, as for instance, conformations with and without hydrogen bonds or with a different number of hydrogen bonds. This effect is corrected at the Gibbs free energy level when including thermal energy and entropy contributions. Considerable compensation of correlation energy and entropy contributions is mainly responsible for the relatively good correspondence of Hartree-Fock energy differences obtained with more extended basis sets and the free enthalpy data at the correlation energy level.