where R(λ) are constraints on the λ i which enable to control the number of the small direct-products in the Smolyak expansion. The usual constraints are defined as L-n+1 ≤ |λ|≤ L where L is the Smolyak parameter and |λ|= n i=1 λ i . Furthermore, the D λ coefficients are (-1) L-|λ| C L-|λ| n-1 and C j i are binomial coefficients. However, in the present study, we have used constraints adapted to a system-bath separation with three Smolyak parameters: (i) L b , a parameter associated to the bath modes (i