The answer in short: Perform a target calculation with volume as target and pressure as target variable instead.
When calculations are made under constant product volume by using the option ‘VT’ of the subroutines TQSETC (Set – Equilibrium – Condition) or TQSTEC (Set – Equilibrium – Condition – When – Stream – Input), ChemApp will make a global minimisation of the Gibbs energy, changing the total pressure in accordance with the given volume. This is, however, a calculation which in rare occasions might fail to converge, for instance when for the given input the volume exhibits a discontinuous change at the appearance or disappearance of phases and the entered product volume lies within the interval of the discontinuity.
If the same situation would appear during a target calculation with volume as target and pressure as target variable, then ChemApp will weigh all numbers on both sides of the discontinuity and a converged solution will be obtained.
Because of these problems related to constant volume calculations, the decision has been made to delete ‘VT’ as a possible option for TQSETC and TQSTEC in future versions of ChemApp. In all application programs where it is currently being used, the option ‘VT’ should be replaced by option ‘V’, such defining a target calculation. Note that this does not affect the subroutine TQGETR (Get – Result) and its option ‘VT’ in any way.
The Programmer’s Manual (Edition 3.2 and later) contains a programming example for this in the Advanced topics section of the appendix (-> ChemApp documentation light/regular).