Band calculates the overlap matrix of the core functions, and this should approximate the unit matrix. When the deviation is larger then the frozen-core overlap criterion the program stops. The default criterion (0.98) is fairly strict. The safest solution is to choose a smaller frozen core. For performance reasons, however, this may not be the preferred option. In practice you might still get reliable results by setting the criterion to 0.8, see the Dependency keyword. For the 5d transition metals, for instance, you can often freeze the 4f orbital, thus reducing the basis set considerably. We strongly advise you to compare these results to a calculation with a smaller core. Such tests can be performed with a smaller unit cell or with a smaller KSPACE parameter.
