The increasing availability of high-quality, experimental data and first-principles calculations creates opportunities for developing more accurate empirical force fields for simulation of proteins. We developed the AMBE-R-FBI5 protein force field by building a high-quality quantum chemical data set consisting of comprehensive potential energy scans and employing the ForceBalarice software package for parameter Optiniization. The optimized' potential surface allows-for more significant thermodynamic, fluctuations away from local minima. In validation studies where simulation rsults are compared to experimental measurements, AMBER-FBI-5 in combination with the updated TIP3P-FB water model predicts equilibrium properties with equivalent accuracy, and temperature dependent properties with significantly improved accuracy, in comparison with published-models. We also discuss the effect of changing the protein force field and water model on the simulation results.