Extensive benchmarking calculations are presented to assess the accuracy of the standard approximate coupled cluster singles and doubles method (CC2) in studying * excited states properties of model protein chains containing a phenylalanine residue, namely capped peptides, whose ground state conformers adopt the prototypical secondary structural features of proteins. First, the dependence with the basis-set of the CC2 excitation energies, CC2 geometry optimizations and amide A region frequencies of the lowest * excited state in a reference system, the N-acetyl-phenylalaninylamide, are investigated and the results are compared with experimental data. Second, at the best level of theory determined, the CC2/aug(N,O,)-cc-pVDZ//CC2/cc-

In Kohn-Sham electronic structure computations, wave functions have singularities at nuclear positions. Because of these singularities, plane-wave expansions give a poor approximation of the eigenfunctions. In conjunction with the use of pseudo-potentials, the PAW (projector augmented-wave) method circumvents this issue by replacing the original eigenvalue problem by a new one with the same eigenvalues but smoother eigenvectors. Here a slightly different method, called VPAW (variational PAW), is proposed and analyzed. This new method allows for a better convergence with respect to the number of plane-waves. Some numerical tests on an idealized case corroborate this efficiency. This work has been recently announced in [3]. arXiv:1711.06529v1 [math.NA] 17 Nov 2017 1. the number N I of PAW functions used to build T I , 2. a cut-off r c radius which will set the acting domain of T I , more precisely: