Sets the lower bound for the variable threshold for screening density matrix
values in the IncDFT procedure. The threshold will begin at this value and
then vary depending on the error in the current SCF iteration until the value
specified by INCDFT_DENDIFF_THRESH is reached. This means this
value must be set lower than INCDFT_DENDIFF_THRESH.
Variable threshold is not used.
Corresponding to a threshold of .
If the default value causes convergence problems, set this value higher
to tighten accuracy. If this fails, set to 0 and use a static threshold.