Résumé
A Smolyak algorithm adapted to system−bath separationis proposed for rigorous quantum simulations. This technique combines asparse grid method with the system−bath concept in a specificconfiguration without limitations on the form of the Hamiltonian, thusachieving a highly efficient convergence of the excitation transitions forthe “system” part. Our approach provides a general way to overcome theperennial convergence problem for the standard Smolyak algorithm andenables the simulation of floppy molecules with more than a hundreddegrees of freedom. The efficiency of the present method is illustrated onthe simulation of H2 caged in an sII clathrate hydrate including two kindsof cage modes. The transition energies are converged by increasing thenumber of normal modes of water molecules. Our results confirm thetriplet splittings of both translational and rotational (j = 1) transitions ofthe H 2 molecule. Furthermore, they show a slight increase of the translational transitions with respect to the ones in a rigid cage.