neqsim: Skill for Claude Code

NeqSim Regression Baseline Management

Preventing silent accuracy drift in a physics engine requires committed baseline values that CI validates on every build.

Why Baselines Matter

A property correlation change that improves methane density by 0.1% might degrade ethane viscosity by 5%. Without baselines, these regressions are invisible until a downstream user reports wrong results months later.

Baseline Workflow

When to Create Baselines

Create baselines before modifying:

• Flash calculation algorithms (flashops/)
• Phase property calculations (phase/, physicalproperties/)
• Component parameter databases (COMP.csv, mbwr32param.csv)
• Mixing rule implementations
• Process equipment calculations that depend on thermodynamic properties
• Cost estimation or mechanical design correlations

Step 1: Capture Current Values

Run the existing code and record results in a JSON fixture file:

@Test
void captureBaseline_SRK_methane_density() {
    SystemInterface fluid = new SystemSrkEos(273.15 + 25.0, 60.0);
    fluid.addComponent("methane", 1.0);
    fluid.setMixingRule("classic");

    ThermodynamicOperations ops = new ThermodynamicOperations(fluid);
    ops.TPflash();
    fluid.initProperties();

    // Record these values as the baseline
    double density = fluid.getDensity("kg/m3");     // e.g., 45.23
    double Cp = fluid.getCp("J/molK");              // e.g., 38.5
    double Z = fluid.getZ();                        // e.g., 0.892

    assertEquals(45.23, density, 0.5, "Methane density at 25C/60bar");
    assertEquals(38.5, Cp, 0.5, "Methane Cp at 25C/60bar");
    assertEquals(0.892, Z, 0.005, "Methane Z-factor at 25C/60bar");
}

Step 2: Choose Appropriate Tolerances

Property Type	Typical Tolerance	Rationale
Density	0.5-1.0% relative	Well-predicted by cubic EOS
Z-factor	0.5% absolute (0.005)	Directly from EOS
Viscosity	2-5% relative	Correlation-dependent, more variable
Thermal conductivity	5-10% relative	Least accurate transport property
Phase fractions	1% absolute (0.01)	Phase split sensitivity varies
Enthalpy/Cp	1-2% relative	Derived from EOS
Bubble/dew point	0.5-1.0 K or 0.5-1.0 bar	Phase boundary sensitivity
Compressor power	1-2% relative	Depends on enthalpy accuracy
Separator compositions	2% relative per component	Depends on K-value accuracy