Building on the insights gained from electronic-structure calculations and from experience obtained with an earlier atomic-level method, we developed an atomic-level simulation approach based on the traditional Buckingham potential with shell model which correctly reproduces the ferroelectric phase behavior and dielectric and piezoelectric properties of KNbO3. This approach now enables the simulation of solid solutions and defected systems; we illustrate this capability by elucidating the ferroelectric properties of a KTa0.5Nb0.5O3 random solid solution.