The choice of basis states in quantum calculations can be influenced b
y several requirements, and sometimes a very natural basis suggests it
self. However often one retreats to a ''merely complete'' basis, whose
coefficients in the eigenstates carry Little physical insight. We sug
gest here an optimal representation, based purely on classical mechani
cs. ''Hidden'' constants of the motion and good actions already known
to the classical mechanics are thus incorporated into the basis, leavi
ng the quantum effects to be isolated and included by small matrix dia
gonalizations. This simplifies the hierarchical structure of couplings
between ''zero-order'' states. We present a (non-perturbative) method
to obtain such a basis-state as solutions to a certain resonant Hamil
ton-Jacobi equation. (C) 1997 American Institute of Physics.