A New Upper Bound for the Site Percolation Threshold of the Square Lattice

The upper bound for the site percolation threshold of the square lattice is reduced from 0.679492 to 0.666894, providing the first improvement since 1995. The bound is obtained by using the substitution method with new computational reductions which make calculations for site models more efficient. The substitution method is applied, comparing the site percolation model on a self-matching lattice to the square lattice site percolation model in a two-stage process.