A hybrid QM/MM method employing real space grids for QM water in the TIP4P water solvents

A novel hybrid quantum mechanical (QM)/molecular mechanical (MM) approach that employs the real-space grids for the QM subsystem is proposed for investigating chemical reactions in an aqueous condensed phase. All of the Hamiltonian matrix elements including electric fields formed by the point charges on MM waters is represented in the real space. Details of the practical implementations are presented. The solute polarization, solvation structure, and the solvation energy of a water are computed, and the results are compared with those obtained by experiments and other QM/MM approaches that used the LCAO basis. It is shown that the real-space grid QM/MM method is adequate and superior for the description of the polarization of QM water in a water solution as well as in the gas phase. Solvation structures of classical water solvents are also properly reproduced by this method. Further, parallelization of the code is implemented on a distributed memory architecture, and it is demonstrated that the real-space grid approach is suitable for the high-performance parallel computing due to the localization of Hamiltonian operations in the real space.