We use the density-matrix renormalization group for transfer matrices to nu
merically calculate impurity corrections to thermodynamic properties. The m
ethod is applied to two impurity models in the spin-1/2 chain, namely a wea
k link in the chain and an external impurity spin, The numerical analysis c
onfirms the field theory calculations and gives explicit results for the cr
ossover behavior. [S0163-1829(99)01606-9].