Miller and Lutz [20] developed a generalized method for describing the pressure dependence of a reaction rate based on direct interpolation of reaction rates specified at individual pressures. In this formulation, the reaction rate is described in terms of the standard modified Arrhenius rate parameters. Different rate parameters are given for discrete pressures within the pressure range of interest. When the actual reaction rate is computed, the rate parameters will be determined through logarithmic interpolation of the specified rate constants, at the current pressure from the simulation. This approach provides a very straight-forward way for users to include rate data from more than one pressure regime.

For a given reaction, then, the user would supply rate parameters for a set of pressures. The set of pressure points for which rate parameters are specified must include at least two different pressures. In such cases the calculation first sums up the rate constants at the bounding pressures and then does the interpolation for the given . In other words, if and are the rate constants at pressure and is the rate constant at pressure , then the net rate at pressure is computed as follows:

Note that this allows use of a negative pre-exponential in the Arrhenius rate specification (although, in general, this is not recommended) as long as the sum (that is, the net rate at a given pressure) doesn't become negative at any temperature in the entire simulation domain at any time. It should also be noted that when the reaction itself is duplicated (with keyword DUP appropriately used) then the net rate is obtained by summing up the individual reaction rates, like any other duplicate reaction. However, usage with DUP may run into trouble due to the negative A-factor.

During a simulation, if the current pressure is within 1% of one of the pressures for which rate constants were provided, then that set of rate parameters will be used directly. However, If the current pressure is in between the pressure points provided, then the rate will be obtained by a linear interpolation of as a function of (natural logarithms). For between and , k is obtained using Equation 3–37 .

(3–37)

If the rate of the reaction is desired for a pressure lower than any of those provided, the rate parameters provided for the lowest pressure are used. Likewise, if rate of the reaction is desired for a pressure higher than any of those provided, the rate parameters provided for the highest pressure are used. If the rate at a given pressure cannot be described by a single set of Arrhenius parameters, more than one set may be provided.

This logarithmic interpolation method can be used as an alternative approach to describing any type of pressure dependence, including the multiple-well, multiple-channel reactions discussed in Multiple-well Multiple-channel Reactions Using Chebyshev Polynomials . It has the advantage of being conceptually straightforward to implement. However, the resolution or accuracy of the pressure dependence will depend on the number of pressure points included for each reaction.