Distributed Multipoles and Energies of Flexible Molecules

In this work we show that energies and distributed multipoles, up to and including rank two, can be accurately determined via a modified Shepard interpolation of ab initio data for small molecules. The molecules considered here are the amino aldehydes, Gly and Ala, which may be typical smaller fragment molecules in certain molecular energy-based fragmentation schemes. The method is general and should be suitable for applications also involving crystal structure prediction, modeling molecular clusters, and Monte Carlo or molecular/reaction dynamics simulations. The configuration space covered by the interpolation includes that sampled by the Gly and Ala peptides in protein crystal structures, i.e., 12 dimensions for Gly: 3 torsion angles (φ, ψ, ω), 5 bond lengths, and 4 bond angles and 15 dimensions for Ala: 4 torsion angles, 6 bond lengths, and 5 bond angles. In this work we also describe a new method of importance, sampling the relevant configuration spaces, and show that it is possible to interpolate "axis free" multipoles.