Let Ω be a C ∞ -smooth bounded domain of R n , n 1, and let the matrix a ∈ C ∞ (Ω; R n 2 ) be symmetric and uniformly elliptic. We consider the L 2 (Ω)-realization A of the operator − div(a∇·) with Dirichlet boundary conditions. We perturb A by some real valued potential V ∈ C ∞ 0 (Ω) and note A V = A + V . We compute the asymptotic expansion of tr(e −tA V − e −tA ) as t ↓ 0 for any matrix a with constant coefficients. In the particular case where A is the Dirichlet Laplacian in Ω, that is when a is the identity of R n 2 , we make the four main terms appearing in the asymptotic expansion formula explicit and prove that L ∞ -bounded sets of isospectral potentials of A are bounded in H 2 (Ω).