The QM subsystem was in all cases modeled using the BLYP exchange-correlation functional, employing the new driver by Bircher and Rothlisberger, and Troullier-Martins norm-conserving pseudopotentials. For GB1 the bonds between the C α and C β of the zinccoordinating histidines were cut and the C α 's were replaced by boundary atoms described by an analytic monovalent pseudopotential (boundary atoms are shown in yellow in Fig. ).