The equation of state (EoS) associated with fluid species is specified in a confusing variety of ways, this unfortunate state of affairs is due to my attempt to maintain backward-compatibility in Perple_X. The association may be influenced by: A) Whether the species is represented by thermodynamic, saturated, or saturated-phase components as specified in the computational option file (the file generated by BUILD). B) Whether the thermodynamic data file identifies the species as a special component in the header of the thermodynamic data file search for "special_component" to determine this (www.perplex.ethz.ch/perplex_thermodynamic_data_file_body.html#Special_components:). C) Whether the EoS flag specified in the thermodynamic data file for the species is > 1. To determine this search for the entry for the species in the data file. The EoS flag follows the species name (www.perplex.ethz.ch/perplex_thermodynamic_data_file_body.html#Data_Entries). ================================================================= Some nomenclature: A composant is a species or phase with the same composition as a component. A saturated component constraint requires that a composant of the saturated component is stable, but the composant is chosen by minimizing the Gibbs energy, e.g., if H2O is chosen as a saturated component, then the corresponding composant may be any phase with the composition H2O. A saturated-phase constraint specifies the composition of a phase, usually a molecular fluid, which is assumed to be stable. Saturated-phase constraints rely on an internal EoS that is specified in the problem definition file by the line 14 ifug EoS for saturated phase This specification identifies the FLUID EoS but, in the case of hybrid fluid EoS, the EoS actually used for the fluid species is determined by hybrid_EoS_*** option. ================================================================= The EoS decision tree based on the component choice (A above) follows. For each the most likely, i.e., recent configuration is specified first: ================================================================= 1) The fluid species is in the thermodynamic composition space, e.g., the species is H2O and H2O is specified as a thermodynamic component: Case 1A: the EoS flag for the species in the thermodynamic data file is > 100, then the EoS is specified by the hybrid_EoS_*** option. This is the case for most recent data files. Eos flag values > 100, < 200, identify specific molecuar species as summarized at www.perplex.ethz.ch/perplex_thermodynamic_data_file_body.html#Special_EoS Case 1B: the EoS flag for the species in the thermodynamic data file is = 10, then the ideal gas is used for the species. Case 1C: if NOT case 1A and NOT case 1B and the species is identified as a special component, then the molecular fluid EoS specified in the computational option file by the line: 14 ifug EoS for saturated phase is used for the species. If the fluid EoS is a "hybrid" EoS, then the EoS actually used for the species is, again, determined by the hybrid_EoS_*** option. To override this behavior set the GFSM option to T or delete the "special_component" section from the thermodynamic data file header AND add an EoS association for the species of interest (i.e., 10 or > 100). Case 1D: the EoS flag for the species is < 10, the species will automatically be rejected from any calculations made with MEEMUM, VERTEX, or CONVEX. This behavior can be overridden by setting the auto_exclude option to F, in which case no volumetric EoS will be associated with the species (i.e., its Gibbs energy will not depend on pressure). Setting auto_exclude to F is NOT recommended. ================================================================= 2) The fluid species is specified as a saturated component, i.e., NOT a saturated-PHASE component: Case 2A: the EoS flag for the species in the thermodynamic data file is > 100, then the EoS is specified by the hybrid_EoS_*** option. This is the case for most recent data files. Case 2B: see Case 1C ================================================================= 3) the fluid species is specified as a saturated-PHASE component: Case 3A: the EoS specified in the computational option file by the line: 14 ifug EoS for saturated phase determines the EoS used for the saturated-PHASE. If this EoS is a hybrid molecular fluid EoS then the EoS specified by the hybrid_EoS_*** option is used for the individual species of the phase. Case 3B: several archaic saturated-PHASE fluid EoS (choices 8, 10, 12, 17, 19, and 20) allow calculation of the properties of carbon-saturated fluids with either H2O and CO2 or H2 and O2 as saturated phase components. When H2O and CO2 are used for this purpose it is possible to trick Perple_X into computing redox phase equilibria by specifying O2 as a saturated component. When this is done, the EoS for the O2 composant must be set to 1, all other O2 composants must be excluded, and the auto_exclude option set to F. This complexity can be avoided by using H2 and O2 as saturated-PHASE components, better still use of generic fluid solution model (GFSM) in the thermodynamic composition space allows a for a completely robust treatment of redox phenomena and graphite precipitation. GFSM cannot be specified by saturated-PHASE constraints and therefore GFSM saturation can only be assured by making the system composition so volatile rich that the fluid is always stable.