We propose a method to determine the locally preferred structure of model liquids. The latter is obtained numerically as the global minimum of the effective energy surface of clusters formed by small numbers of particles embedded in a liquidlike environment. The effective energy is the sum of the intracluster interaction potential and of an external field that describes the influence of the embedding bulk liquid at a mean-field level. Doing so we minimize the surface effects present in isolated clusters without introducing the full blown geometrical frustration present in bulk condensed phases. We find that the locally preferred structure of the Lennard-Jones liquid is an icosahedron, and that the liquidlike environment only slightly reduces the relative stability of the icosahedral cluster. The influence of the boundary conditions on the nature of the ground-state configuration of Lennard-Jones clusters is also discussed. (C) 2003 American Institute of Physics.