The system parameter PENTLY is only required when constraint equations are used to apply boundary conditions across two or more separate bodies in static or dynamic analyses and when the Frontal solver is being used.

When using constraint equations, the diagonal elements of the coefficient stiffness matrix corresponding to the Lagrange multipliers are zero. If the constraint equations are processed as they occur (arbitrarily) in the structure, this will result in a singular matrix and solution will not be possible. To circumvent this problem, therefore, the constraint equations are always solved subsequent to the structural elements attached to each constraint to ensure that the total stiffness coefficient of each degree of freedom is present and the diagonal is negative definite (the warning messages associated with these negative values being suppressed in this case).

In considering the case of a constraint equation connecting the degrees of freedom of two separate bodies there will not be a stiffness term associated with the equation even if the constraints are processed last, and a singular matrix will, again, result. The use of the PENTLY parameter simply places a one degree of freedom spring in parallel with each constraint equation to give such a stiffness - the spring value is necessarily small to avoid ill-conditioning but other than this, it has no effect except to enable the solution to proceed.

Hence, PENTLY may be usually specified as unity from File> Model properties> LUSAS system variables tab.