Having had my say about the AS/AD framework in a number of articles, I
will be brief. Clearly, Kevin is correct--the AD curve can be derived
from IS/LM, and given assumptions, it can be justified. But IS/LM is a
model of goods/money market equilibrium--so what one gets from that
derivation is a goods market equilibrium curve, not an aggregate demand
curve. However, then, in AS/AD analysis, one adds an AS curve, and talks
about disequilibrium as well as equilibrium. Yes, the math for the
equilibrium points can work out, but the discussion of disequilibrium
adjustment forces in the texts is generally problematic, if not
downright wrong.

Dave Colander