MFCC-downhill simplex method for molecular structure optimization

In this paper, a MFCC-downhill simplex method is proposed to study binding structures of small molecules or ions in large molecular complex systems. This method employs the Molecular Fractionation with Conjugated Caps (MFCC) approach in computing inter-molecular energy and implements the downhill simplex algorithm for structural optimization. The method is numerically tested on a system of [KCp(18-crown-6)] to optimize the position of the potassium cation in a fixed coordination sphere. The result of MFCC-downhill simplex optimization method shows good agreement with both the crystal structure and with the full-system downhill simplex optimized structure. The effect of the initial structure of the simplex and the method/basis-set levels of the quantum chemical calculation on the MFCC-downhill simplex optimization are also discussed. This method should be applicable to structure optimization of large complex molecular systems such as proteins or other biopolymers.