QM-MM interactions in simulations of liquid water using combined semi-empirical/classical Hamiltonians

Well-established semi-empirical quantum-mechanical methods such as AM1, PM3 and MNDO poorly describe hydrogen-bonding interactions. The recently developed OM1, OM2 and OM3 methods, which include explicit corrections in the one-electron terms of the Hamiltonian to counteract for the neglect of orthogonalization effects, show an improved description of hydrogen bonding. In the current work we compare the performance of the OMx models on the one hand and the AM1, PM3 and MNDO methods (as well as the SCC-DFTB method) on the other when used in combination with a classical force field in QM/MM calculations on a water dimer in the gas phase, and in QM/MM simulations of a QM water 'solute' in liquid MM water. It is found that when using the OMx methods to describe the QM water molecule, values for QM-MM interaction energies and the molecular dipole moment of the QM solute are significantly closer to experiment than when using the other semi-empirical methods. In addition, the compatibility of OM3 with the classical Hamiltonian improves upon explicitly including electronic polarization in the MM subsystem.