  Standard methods in computer model calibration treat the calibration
parameters as constant throughout the domain of control inputs. In many
applications, systematic variation may cause the best values for the
calibration parameters to change between different settings. When not accounted
for in the code, this variation can make the computer model inadequate. In this
article, we propose a framework for modeling the calibration parameters as
functions of the control inputs to account for a computer model's incomplete
system representation in this regard while simultaneously allowing for possible
constraints imposed by prior expert opinion. We demonstrate how inappropriate
modeling assumptions can mislead a researcher into thinking a calibrated model
is in need of an empirical discrepancy term when it is only needed to allow for
a functional dependence of the calibration parameters on the inputs. We apply
our approach to plastic deformation of a visco-plastic self-consistent material
in which the critical resolved shear stress is known to vary with temperature.
