  One may impose to a system with spontaneous broken symmetry, boundary
conditions which correspond to different pure states at two ends of a sample.
For a discrete Ising-like broken symmetry, boundary conditions with opposite
spins in two parallel limiting planes, generate an interface and a cost in free
energy per unit area of the interface. For continuum symmetries the order
parameter interpolates smoothly between the end planes carrying two different
directions of the order parameter. The cost in free energy is then proportional
to $L^{d-2}$ for a system of characteristic size L. The power of $L$ is related
to the lower critical dimension, and the coefficient of this additional free
energy vanishes at the critical temperature. In this note it is shown within a
loop expansion that one does find the expected behavior of this twist free
energy. This is a preamble to the study of situations where the broken
continuum symmetry is believed to be more complex, as in Parisi's ansatz for
the Edwards-Anderson spin glass.
