  Using double parabola approximation for a single Bose-Einstein condensate
confined between double slabs we proved that in grand canonical ensemble (GCE)
the ground state with Robin boundary condition (BC) is favored, whereas in
canonical ensemble (CE) our system undergoes from ground state with Robin BC to
the one with Dirichlet BC in small-$L$ region and vice versa for large-$L$
region and phase transition in space of the ground state is the first order.
The surface tension force and Casimir force are also considered in both CE and
GCE in detail.
