[en] The determination of the ideal MHD stability properties of plasma containment devices is never complete without the investigation of the impact that can be had by the class of instability that can displace the plasma-vacuum interface. These free-boundary modes impose in many cases the severest restrictions on the plasma beta that a configuration can achieve and/or the toroidal plasma current that can flow within it. In order to examine the free-boundary stability of a plasma, the potential energy contribution of the vacuum region that surrounds the plasma must be added to the internal contribution of the plasma itself. In two-dimensional (2D) plasmas, two methods have been developed to tackle the vacuum contribution to the energy principle. One scheme uses a Green's function technique which is particularly convenient when the conducting wall is placed at infinity. An alternative scheme is to treat the vacuum region as a pressureless and shearless pseudo-plasma. The main advantage of this scheme is that the structure of the problem in the vacuum region is identical to that in the plasma region. A detailed analysis and comparison of the two methods in 1D axisymmetric and helically symmetric geometry has been discussed by Gruber and Rappaz (1985) and by Gruber et al. (1981). We have generalized the vacuum treatment as a pseudo-plasma to three dimensional (3D) geometry and implemented it in the TERPSICHORE ideal MHD stability code, which was previously limited only to the study of internal structures. (author) 7 refs., 2 figs