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Using an example of a platform chemical 3-hydroxypropionic acid,we comprehensively illustrate a general process of fitting all-atom molecular mechanics force field parameters based on quantum mechanical calculations and experimental thermodynamic data.For common organic molecules with free dihedral rotations,the present force field format is comprised of the usual bond stretching,angle bending,proper and improper dihedral rotation,and 1-4 scaling pair interactions.An extra format of 1-n scaling pair interaction is introduced when a specific intramolecular rotation is strongly hindered.We detail how the determination of all intramolecular force field parameters can be decoupled by systematically generating characteristic configurations.The intermolecular Van der Waals parameters are initially taken from the literature data but adjusted to obtain a better agreement between the molecular dynamics simulation results and the experimental observations.By randomly choosing the molecular configurations from molecular dynamics simulation and comparing their energies computed from force field parameters and quantum mechanics,the force field parameters can be verified self-consistently.The introduced systematic approach can be applied to force field development for organic molecules with special functional groups for which no suitable molecular mechanics parameters in the current force field are available.[1] We present a rigorous Ewald summation formula to evaluate the electrostatic interactions in two-dimensionally periodic planar interfaces of three-dimensional systems.By rewriting the Fourier part of the summation formula of the original Ewald2D expression with an explicit order N2 complexity to a closed form Fourier integral,we find that both the previously developed electrostatic layer correction term and the boundary correction term naturally arise from the expression of a rigorous trapezoidal summation of the Fourier integral part.We derive the exact corrections to the trapezoidal summation in a form of contour integrals offering precise error bounds with given parameter sets of mesh size and system length.Numerical calculations of Madelung constants in model ionic crystals of slab geometry have been performed to support our analytical results.[1] Both the algorithms for electrostatic interaction at interfaces and force field development methods have been incorporated into the homemade software for molecular simulation written in the past two years.