Pressure effects on the ground-state energy and molecular conformation of methane, ammonia and water are simulated through a molecular confinement model whereby an isolated molecule is caged in a hard spherical box of varying radius. A self-consistent ab initio calculation subject to Dirichlet boundary conditions is performed for each cage radius using a submininial Floating Spherical Gaussian Orbital representation. Maximum bond compressibilities for water (beta approximate to 4 x 10(-4) GPa(-1)), ammonia (beta approximate to 8 x 10(-4) GPa(-1))and methane(beta approximate to 1.3 x 10(-3) GPa(-1)) are predicted at P = 40, 25 and 15 GPa, respectively. The pressure-induced changes in the dipole moment of water and ammonia are also discussed. (C) 2004 Elsevier B.V. All rights reserved.