Simple one-electron quantum capping potentials for use in hybrid QM/MM studies of biological molecules

Calculations demonstrate that with a minor modification conventional ab initio effective potentials can be employed in place of link atoms to truncate quantum regions in hybrid quantum mechanics/molecular mechanics calculations. Simple quantum capping potentials are formed by replacing excess valence electrons in conventional effective potentials by spherical shielding and Pauli terms chosen to duplicate all-electron molecular structures and charge distributions. Tests involving truncated histidine show errors in charge and protonation energy to be reduced as compared to the link atom approach. Because of the use of conventional effective potential expansions, this approach can be implemented with minimal or no program modifications. Indeed, in its simplest form it requires the addition of only a single Gaussian and adjustable parameter to a conventional effective potential expansion. The parametrization requires little effective potential expertise or effort.