Quantum calculations on the open gate and the immediately surrounding region of the Kv1.2 voltage gated potassium channel show that water adopts two conformations there, one when the ion is present, and another with no ion present. An energy minimum at the gate with no ion allows an ion to enter from the solution; from the gate it repels an ion in the channel pore cavity into the selectivity filter (essentially the “knock-on” mechanism). The pore cavity minimum can then be occupied by the ion moving from the gate minimum, leaving it available for anion from bulk, so the cycle continues. The arrangement of water, and the ion hydration, defines the gate energy minimum, and this in turn depends on the highly conserved prolines, especially P407 (2A79 pdb numbering), in the PVPV sequence in the gate. Absent an energy minimum at the gate an ion entering from solution would be repelled back into solution by the ion in the cavity. Furthermore, the ion remains hydrated by 8 water molecules, at least 1 more than in bulk, as it passes through the gate, based on optimizations with the ion at three different positions: 1) at the gate position nearest P407 2) 2A above 3) 2A below, this position. The open gate maintains very nearly the same protein atom positions throughout. Additional calculations will be required to determine the exact interaction energy with an ion in the cavity, and the amount of water that is present in the cavity. Optimizations of the system configuration were done at HF/4-22GSP level, with 692 atoms (693 with the ion). Energy was determined using both B3LYP/6-31G∗ and bvp86/6-31G∗, but the system was too large for free energy determination.