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 used at the HF and MP2 levels of theory for geometry optimizations of the global, gamma(L), (phi = -75 degrees, Psi = +75 degrees) and the second 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 HF/6-311++G(d,p) energy difference converges erroneously toward zero. The 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 that 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 conformers.