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This keyword data block is used to enable printing of isotopic fractionation factors, referred to as alphas, to the output file. A Basic function defined in CALCULATE_VALUES is used to calculate the fractionation factor from the current isotopic composition of species or phases and an analytical expression for a fractionation factor is evaluated by a definition in NAMED_EXPRESSIONS. These two values and related data are printed in the output file under the heading “Isotope Alphas”. The ISOTOPE_ALPHAS data block is used in the database file iso.dat and is unlikely to be used in any other context.
calculate_values_function --The name of a calculate values function (CALCULATE_VALUES data block) that evaluates a fractionation factor based on the isotopic compositions of species or phases.
named_expression --The name of a named expression (NAMED_EXPRESSIONS data block) that evaluates an analytical expression for a fractionation factor between species or phases.
This keyword data block is used to implement the treatment of isotopes as individual thermodynamic components (Thorstenson and Parkhurst, 2000, 2004). If R is defined to be the ratio of the number of moles of the minor isotope to the number of moles of the predominant isotope in a species or phase, then the fractionation factor, or alpha, is the ratio of R in one species or phase to R in another species or phase. In the Example data block given in this section, the fractionation factors are calculated for deuterium (D) and tritium (T) between hydroxide ion and liquid water. Analytical expressions for fractionation factors are defined in the database through the use of the NAMED_EXPRESSIONS data block and are incorporated into equilibrium constants for species and phases in SOLUTION_SPECIES and PHASES data blocks. The fractionation factor based on solution and phase composition can be calculated by Basic functions that are defined in the CALCULATE_VALUES data block. At equilibrium, fractionation factors derived from the composition of the solution and other phases should equal the fractionation factor derived from the named expression, just as the ion-activity product of a phase should equal the equilibrium constant at equilibrium. This correspondence between composition-derived and analytical fractionation factors is printed in the output file under the heading “Isotope Alphas”. The ISOTOPE_ALPHAS data block only defines quantities to print and by itself does not affect the equilibrium distribution of species in a simulation.
The use of CALCULATE_VALUES functions to evaluate isotope alphas may be expensive in terms of computer time. If -isotope_alphas is true (PRINT data block), all isotope alphas defined in the database or the input file are evaluated for each reaction calculation, even if the relevant isotopes are not in the reaction system. The Basic function SUM_SPECIES, which is used in many of the isotope alpha calculations, is especially time consuming. Minimizing the number of isotope alphas that are defined, minimizing the use of the SUM_SPECIES function in the CALCULATE_VALUES programs, and setting -isotope_alphas false in a PRINT data block will decrease execution times for isotopic calculations.
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