How would I modify calculation to have value = zero (0) after reaching a minimum.

I have the following formula:

=IF($C$5=0,$C$3*EXP(-$C$4*B15),IF(D14=($C$6+0),C14*EXP(-$C$6*(B15-(B15-1))),$C$3*(1+$C$5*$C$4*B15)^(-1/$C$5)))

I want to modify the formula whereby the value = 0 upon reaching a minimum limit of a fixed value in a single cell. For example, value = 0 if value < fixed value.

assuming your "fixed" value is in A1....
=if(IF($C$5=0,$C$3*EXP(-$C$4*B15),IF(D14=($C$6+0),C14*EXP(-$C$6*(B15-(B15-1))),$C$3*(1+$C$5*$C$4*B15)^(-1/$C$5)))<A1,0,IF($C$5=0,$C$3*EXP(-$C$4*B15),IF(D14=($C$6+0),C14*EXP(-$C$6*(B15-(B15-1))),$C$3*(1+$C$5*$C$4*B15)^(-1/$C$5))))