An effective Hamiltonian for the ferroelectric transitions inKNbO3is constructed from first-principles density-functional-theory total-energy and linear-response calculations through the use of a localized, symmetrized basis set of “lattice Wannier functions.” The detailed description provided here provides the foundation for the use of this effective Hamiltonian in classical Monte Carlo, classical molecular dynamics and quantum mechanical simulations, as well as guidance for the construction of effective Hamiltonians for related systems. ©1998 American Institute of Physics.