# HydroGeoSphere/Absolute Concentration and Temperature as Primary Variables

The following instruction can be used to enable a density-dependent flow and transport solution. In this case, fluid density is directly calculated from individual species concentrations and temperature. The allowed species are ${\displaystyle Na^{+},K^{+},Ca^{2+},Mg^{2+},Cl^{-},SO_{4}^{2-},CO_{3}^{2-}}$ and ${\displaystyle HCO_{3}^{-}}$ and fluid temperature ${\displaystyle T}$, which have to be defined within the solute definition block (See Section 5.6.3.1). Note that the units must be mg l-1 and °C, respectively.

## Use Pitzer model

With this instructions, fluid density is calculated from species concentrations using Pitzer’s ion interaction model. Note that this method can be very time-consuming because fluid density is calculated by iterating between density and molality. The default is a faster and non-iterative empirical approach.

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## Picard convergence criteria

1. toldens The minimum absolute difference in head, concentration and temperature for convergence of the non linear Picard loop.
2. maxitdens The maximum number of iterations for the Picard loop. If the maximum number of iterations is reached without obtaining convergence, the time step is cut in half and the loop is started over.

Similar to the use of the ”Density-dependent transport” instruction when using relative concentration, the above instruction assigns new values to the convergence criteria. The defaults are 1.0 × 10-5 and 100, respectively.

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