Energetics and structure of glycine and alanine based model peptides: Approximate SCC-DFTB, AM1 and PM3 methods in comparison with DFT, HF and MP2 calculations

We calculate relative energies and geometries of important secondary struct ural elements for small glycine and alanine based polypeptides containing u p to eight residues. We compare the performance of the approximate methods AM1, PM3 and self-consistent charge, density-functional tight-binding (SCC- DFTB) to density-functional theory (DFT), Hartree-Fock (HF) and MP2. The SC C-DFTB is able to reproduce structures and relative energies of various pep tide models reliably compared to DFT results. The AMI and PM3 methods show deficiencies in describing important secondary structure elements like exte nded, helical or turn structures. The discrepancies between different ab in itio (HF, MP2) and DFT (B3LYP) methods for medium sized basis sets (6-31G*) also show the need for higher level calculations, since systematic errors found for small molecules may add up when investigating longer polypeptides . (C) 2001 Elsevier Science B.V. All rights reserved.