This presentation focuses on our recent efforts to develop improved techniques to calculate electrostatic energies and forces involving systems with explicit multipole moments and those involving complex QM/MM systems.  An overview of new Ewald based methods that have been developed for the CHARMM program will be highlighted. We will demonstrate our efficient arbitrary-order implementation of Coulombic multipole interactions which, in contrast to common implementations, which uses spherical harmonics and a novel internal coordinate system.  Following the work of Sagui, Pedersen and Darden, we introduce particle mesh Ewald terms for spherical harmonic multipoles, allowing most of the work to be performed in reciprocal space.  The internal coordinate system and spherical multipole basis allows for rapid evaluation of the remaining real-space energies and forces.  Combining these real- and reciprocal-space treatments, we can greatly reduce the overhead associated with anisotropic force field terms. For ab initio QM/MM system, we will present our efforts for improved periodic boundary  electrostatics.  Recently, the Quantum Ewald Mesh Method (J. Chem. Phys. 136, 114112 (2012); http://dx.doi.org/10.1063/1.3694829) was developed for fast evaluation of the electrostatics in gas-phase QM calculations within Q-Chem.  In this work, we extended it to QM/MM calculations with a periodic boundary condition interfacing with CHARMM, where the electronic charge density is included explicitly.

Recent Ewald techniques for the efficient calculation of multipole moment interactions and complex QM/MM systems

246th National Meeting (2013)

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