The DF-LCCSD(T0) correction of the φ/ψ force field dihedral parameters significantly influences the free energy profile of the alanine dipeptide

The conformational behavior of small peptides is mostly dictated by backbone rigidity, in which the φ/ψ torsions seem to play the most important role. We show that ab initio-based corrections of the torsion parameters in the FF-FOM force field determined by the DF-LCCSD(T0) method significantly influence the quality and minimum localization on the 2D free energy surface of the alanine dipeptide (AD) along the φ and ψ coordinates of the backbone torsion angles. The populations of the individual conformers are in good agreement with the experimental results published recently on the AD through an analysis of the amide III band and the Raman skeletal vibrations.