The effects of basis set and blocking groups on the conformational energies of glycyl and alanyl dipeptides A Hartree-Fock and MP2 study

Abstract

We present the results of high level ab initio molecular orbital calculations on glycyl and alanyl dipeptides. The results of calculations on the low energy conformers of the methyl-blocked analogs at the MP2/TZVP//HF/6-31G ∗∗ level of theory have previously been reported by two of the authors. In this paper, we examine the effect of carrying out the geometry optimizations of the three methyl-blocked glycyl dipeptide conformers using a larger TZVP basis set followed by an MP2 single-point calculation with that basis set. The resulting geometries and energies were essentially the same as those obtained from optimization with the smaller 6-31G ∗∗ basis set followed by an MP2 single-point calculation with the TZVP basis set. This study then provides additional support for the use of the smaller 6-31G ∗∗ quantum mechanical basis set for the geometry optimizations, since it yields essentially the same structures as the larger TZVP basis set at a reduced computational cost. We also carried out MP2/TZVP//HF/6-31G ∗∗ calculations on the hydrogen-blocked analogs of both dipeptides, so that we might make a more direct comparison with energies previously reported by Head-Gordon and co-workers. The two earlier studies yielded fairly similar results for the two dipeptides, with the exception of glycyl dipeptide where there was an ~0.9 kcal mol −1 difference in the relative energies determined for the C5 conformation. The calculations reported here reveal that this difference was caused by the choice of different blocking groups (hydrogen vs. methyl), rather than the choice of quantum mechanical basis set.