  We consider a rotating Bose-Einstein condensate in a square optical lattice
in the regime in which the Hamiltonian of the system can be mapped onto a
Josephson junction array. In an approximate scheme where the couplings are
assumed uniform, the ground state energy is formulated in terms of the vortex
configuration. The results are compared with experimental results and also
previously reported results for frustrated XY model. We also show that vortex
configuration is robust with respect to change of couplings and therefore the
results remain valid when we consider more realistic model with non-uniform
couplings.
