  Small peptides are model molecules for the amino acid residues that are the
constituents of proteins. In any bottom-up approach to understand the
properties of these macromolecules essential in the functioning of every living
being, to correctly describe the conformational behaviour of small peptides
constitutes an unavoidable first step. In this work, we present an study of
several potential energy surfaces (PESs) of the model dipeptide HCO-L-Ala-NH2.
The PESs are calculated using the B3LYP density-functional theory (DFT) method,
with Dunning's basis sets cc-pVDZ, aug-cc-pVDZ, cc-pVTZ, aug-cc-pVTZ, and
cc-pVQZ. These calculations, whose cost amounts to approximately 10 years of
computer time, allow us to study the basis set convergence of the B3LYP method
for this model peptide. Also, we compare the B3LYP PESs to a previous
computation at the MP2/6-311++G(2df,2pd) level, in order to assess their
accuracy with respect to a higher level reference. All data sets have been
analyzed according to a general framework which can be extended to other
complex problems and which captures the nearness concept in the space of model
chemistries (MCs).
