Anharmonic vibrational spectroscopy of glycine: Testing of ab initio and empirical potentials

The second-order Moller-Plesset ab initio electronic structure method is us ed to compute points on the potential energy surface of glycine. Some 50 00 0 points are computed, covering the spectroscopically relevant regions, in the vicinity of the equilibrium structures of the three lowest-lying confor mers of glycine. The vibrational states and spectroscopy are computed direc tly from the potential surface points using the correlation corrected vibra tional self-consistent field (CC-VSCF) method, and the results are compared with experiment. Anharmonic effects and couplings between different vibrat ional modes that are included in the treatment are essential for satisfacto ry accuracy. The following are found: (1) The spectroscopic predictions fro m the ab initio potential are in very good accord with matrix experiments. (2) Theory agrees even more closely with spectroscopic data for glycine in He droplets, where environmental effects are much weaker than in the matrix . This suggests that errors in the ab initio potential are smaller than rar e-gas matrix effects. (3) The accuracy of the ab initio potential is. by th is spectroscopic test, much superior to that of OPLS-AA, a state-of-the-cut empirical potential. The relative failure of the empirical potential is du e to its inability to describe: details of the hydrogen-bonded interactions , and is most critical in one of the glycine conformers where such interact ions play an especially important role.