Electronic excitations of glycine, alanine and cysteine conformera from first-principles calculations

The electronic and optical properties are studied for three conformers of amino acid molecules using gradient-corrected (spin-) density functional theory within a projector-augmented wave scheme and the supercell method. We investigate single-particle excitations such as ionization energies and electron affinities as well as pair excitations. By comparing eigenvalues resulting from several local and nonlocal energy functionals, the influence of treatment of exchange and correlation is demonstrated. The excitations are described within the Δ-self-consistent field method with an occupation number constraint to obtain excitation energies and Stokes shifts. The results are used to also discuss the optical absorption properties. In contrast to the lowest single-and two-particle excitation energies, remarkable changes are found in absorption spectra in dependence on the conformation of the molecule geometry.