In this work, we model the optical constants of three organic materials using a modified Lorentz oscillator model. We have analysed the impact of the choice of the objective or merit function, which is employed for model parameter estimation, on the modelling of the optical constants. The objective function for model parameter determination should be chosen in such a manner that discrepancies between the experimental and calculated values for both parts of the index of refraction are minimized at the same time. We have found that the choice of objective function strongly influences the existence of multiple solutions. By combining a global optimization method, an appropriately chosen objective function and a modified oscillator model, we obtain good agreement with the experimental data for three organic materials.