We numerically investigate passive optical sorting of plasmon nanoparticles and explore the trajectories of nanoparticles in sorting fields with the optimal wavelength and the optimal beam shape. The optimal wavelength is determined from speed difference to temperature increase ratio of the system. The optimal beam shape for arbitrary size of sorting region is determined using the force optical eigenmode (FOEi) method. Study of deflection trajectories of nanoparticles passing through the optimised field in microfluidic channel scenario demonstrates the feasibility of the method and sets the optimal fluid speed in microfluidic channel to fully utilise the potential of optimised sorting field. The method is demonstrated in specific case of gold nanoparticles of radius 40 nm and 50 nm respectively.