Distributed multipole moments in atomistic force fields. implementation and applications

The accuracy of atomistic force fields depends on the complexity of the interatomic potential function as well as on the parametrization of the potential. In conventional
force fields, the electrostatic potential is represented by atom-centered point charges. Point charges can be understood as the first term of multipole expansions, which converge with increasing number of terms towards the accurate representation of the molecular potential given by the electron density distribution.
Here, the distributed multipole analysis (DMA) is used to obtain atomic multipole moments. The accuracy of distributed multipole potentials is tested for several molecules and compared to point charge potentials. The investigation is focused on convergence of the multipole expansion and conformational dependence. Energies and forces required for molecular dynamics (MD) simulations with atomic multipole
potentials are implemented into the CHARMM program. Important points to consider for the implementation are the orientation of the multipole moments and the conformational dependence of multipole parameters.
The implementation is applied to different systems: The splitting of the infrared (IR) absorption band for photodissociated CO in Myoglobin is analyzed comparing different multipole models for CO. A relationship is established between the IR frequency and the CO orientation in the binding pocket. The experimental IR spectrum of CO
in amorphous ice is reproduced using multipole potentials for CO and water. The relationship between infrared frequencies and ice structures is analyzed. Furthermore,
atomic multipole moments are applied to methane and CO clathrate hydrates. Lattice modes are calculated and compared to experiment. The influence of different guest
molecules on lattice modes and structure is characterized.