The PASSIVE Modifier
Definitions may be specified as PASSIVE, in which case they will be blocked from differentiation with respect to system variables in the formation of the global Jacobian matrix. In strongly nonlinear systems, this sometimes prevents pathological behavior, at the expense of slower convergence.
Viscosity = Passive(3.02*exp(-5*Temp))
The derivative of Viscosity with respect to Temp will be forced to zero, instead of the true value (-5)*3.02*exp(-5*Temp).