We investigate a fully frustrated Ising model with four nearest-neighbor interaction constants on a square lattice. We obtain an exact form of the partition function by using a decimation transformation and a dual transformation. The partition function is then expressed as a product of two partition functions each of which is the one of the Ising model with a nearest-neighbor interaction constant on a square lattice. Special systems with two interaction constants are studied in detail and phase transitions for these systems are discussed.
The spin-spin correlation function in the ground state of some two-dimensional fully frustrated spin models are investigated. The correlations are long ranged and decay as $\frac{1}{{r}^{\ensuremath{\eta}}}$, where
r is the distance between the two spins and $\ensuremath{\eta}=\frac{1}{2}$. This result is true for both the square and triangular fully frustrated lattices, thus suggesting that these models form a universality class. It is shown that the fully frustrated Ising model on a square lattice can be mapped exactly into a special Baxter model. The spin-spin correlation function of this Baxter model can be calculated exactly. The ground state of the fully frustrated square Ising model corresponds to the free fermion point of the
F model and therefore, through duality, to the decoupling point of the critical Baxter line. The relationship between the
F model above the critical temperature and the Gaussian model above the multicritical point is discussed.