Core-ionized and core-excited states of macromolecules

Abstract

We report the calculation of core-ionized states of small organic compounds and macromolecular systems in the framework of a new method based on the local self-consistent field (LSCF). This new theoretical scheme avoids the variational collapse of the empty core orbital (CO) of the core-excited states and ensures the orthogonality between the ground state and the excited states. Compared to experimental data and other theoretical methods, accurate carbon 1s ionization energies using the Boys-Foster (BF) localization criterion for the determination of the CO and the PBE0/6-311++G**//B3LYP/ 6-311++G** level of theory was obtained to calculate both the ground and excited states. The macromolecular systems, a sequence of 15 alanine amino acids in both α-helix and β-sheet conformations, are computed using hybrid quantum mechanics/molecular mechanics (QM/MM) method within the LSCF/MM framework. The results show a weak impact of the MM surrounding in the alanine polypeptides cases compared with that of previous studies based on electrically charged residue, such as glutamate in the crambin protein.