A semiclassical estimate of exponentially decaying tails of energy eigenstates is used to tailor grids in configuration space for specific quantum molecular dynamics computations. The estimate is demonstrated to accurately describe the eigenstate tails of constrained (T-shaped) rare gas triatoms, permitting construction of highly efficient basis sets for accurate numerical simulation. The algorithm is designed to demand minimal exploration of configuration space, and to provide considerable savings in computation time in the case of many degrees of freedom.