Commutator relations are used to investigate the spectra of Schrödinger Hamiltonians, H = −∆ + V (x) , acting on functions of a smooth, compactHere ∆ denotes the Laplace-Beltrami operator, and the real-valued potential-energy function V (x) acts by multiplication. The manifold M may be complete or it may have a boundary, in which case Dirichlet boundary conditions are imposed.It is found that the mean curvature of a manifold poses tight constraints on the spectrum of H. Further, a special algebraic rôle is found to be played by a Schrödinger operator with potential proportional to the square of the mean curvature:where ν = d + 1, g is a real parameter, andwith {κ j }, j = 1, . . . , d denoting the principal curvatures of M . For instance, by Theorem 3.1 and Corollary 4.4, each eigenvalue gap of an arbitrary Schrödinger operator is bounded above by an expression using H 1/4 . The "isoperimetric" parts of these theorems state that these bounds are sharp for the fundamental eigenvalue gap and for infinitely many other eigenvalue gaps.