CAPE-OPEN

I try to calculate the dew point by 'getEquilibrium' function and easy 'for recycle' script in matlab unit-op. I find that the calculated dew temperature is 0.001 high than the actual dew point.
For example, if the dew point is -23.333℃, when calculated by the 'getEquilibrium' function, the phase is also vapor in -23.334℃.

Can you provide the script? Which property package are you using?

Understand, but I would expect the thermodynamic to return a vapor + liquid (or just liquid) at T < Tbub for any P at which the phase boundary is not retrograde. The simple test I suggested would check whether this is the case.