A new optimization method for the determination of classical force fields. Application to the united atom force field of short alkanes

A new optimization scheme based on the steepest descend method is proposed to determine the force field parameters of pure compounds by considering vapor-liquid equilibrium data as the target properties to be optimized. Following the work of Bourraseau et al. [J. Chem. Phys.,118, 3020-3034 (2003)], we calculate the gradient of the objective function by using the fluctuation formulas applied to liquid coexistence density, vapor pressure and vaporization enthalpy. We suggest a new procedure to determine the optimal step length that enables to compute the parameters of the next iteration of the optimization. The new method is employed to the united atom force field parameters for the n-alkanes and a significant improvement is obtained for the description of vapor pressures.