PEPTIDE MODELS .15. THE EFFECT OF BASIS-SET SIZE INCREASE AND ELECTRON CORRELATION ON SELECTED MINIMA OF THE AB-INITIO 2D-RAMACHANDRAN MAP OF FOR-GLY-NH2 AND FOR-L-ALA-NH2

A total of eleven basis sets from 3-21G to 6-3111++G(d,p) have been us ed at the HF and MP2 levels of theory for geometry optimizations of th e global, gamma(L), (phi = -75 degrees, Psi = +75 degrees) and the sec ond lowest, beta(L), (phi = -150 degrees, Psi = +150 degrees) minimum energy conformations of the L enantiomer of HCO-NH-CH(CH3)-CO-NH2. The results showed that due to fortuitous cancellation of correlation and basis set effects, the HF/3-21G energy-difference of these conformers agrees well with the MP2/6-311++G(d,p) energy difference, while the H F/6-311++G(d,p) energy difference converges erroneously toward zero. T he other legitimate conformers were optimized at the HF/3-21G, HF/6311 ++G(d,p), and MP2/6-311++G(d,p) levels of theory. The results showed t hat one of the minima disappeared at HF/6-311++G(d,p) and one more of the minima did not occur at the MP2/6-311++G(d,p) level of theory. The correlation and basis set effects stabilized the higher energy confor mers.