Abstract
Cooperative polarisation in ice Ih is illustrated in clusters of eight molecules using local density functional calculations where the exchange-correlation functional is a modified Ceperley-Alder expression. These parameter-free total energy calculations support the four-point charge/polarisable point dipole model of Kozack and Jordan and indicate that cooperative polarisation has a profound effect, not simply increasing the cohesive energy, but also making bond breaking disproportionately difficult.