Let ${K_{t}}_{t>0}$ be the semigroup of linear operators generated by a Schrödinger operator -L = Δ - V with V ≥ 0. We say that f belongs to $H¹_{L}$ if $||sup_{t>0} |K_{t}f(x)| ||_{L¹(dx)} < ∞$. We state conditions on V and $K_{t}$ which allow us to give an atomic characterization of the space $H¹_{L}$.