where F → → ( a ) = C → → is the constraint, and a i j ( a i j ≥ ≥ 0 ) are the elements in the adjacency matrix, a i j > 0 if and only if there is a link between node i and node j. Θ Θ ( a i j ) is a step function with Θ Θ ( a i j ) = 1 if x > 0 , and Θ Θ ( a i j ) = 0 if x = 0 . The auxiliary fields h i j and r i j have been introduced as analogy to the bath in classical mechanics.