Abstract
We employ two-body and three-body bond-orientational order-parameters, in conjunction with non-Boltzmann sampling to calculate the free energy barrier to nucleation of crystalline phases of water. We find that, as the coupling between the successive peaks of the direct correlation function increases, the free energy barrier to nucleation decreases. On this basis we explain the important parameters that govern the nucleation rate involving crystalline phases of water in different homogeneous and inhomogeneous environments, giving a "unified picture" of ice nucleation in water.