 Still later in 1967, C. P. Yang generalised Sutherland's solution to an exact solution for square-lattice ice-type models in a horizontal electric field.In 1969, John Nagle derived the exact solution for a three-dimensional version of the KDP model, for a specific range of temperatures.For such temperatures, the model is "frozen" in the sense that (in the thermodynamic limit) the energy per vertex and entropy per vertex are both zero.