How to add value of 5e^(-25/2t)u(t) to a voltage source in LTSpice?
Use a voltage b-source with this:

V = 5*exp(-12.5*time)


U(t) is not needed because it is a step input and goes to 1 as soon as the simulation starts (at t=0 the b-source is considered on)

If you wanted a time offset (like 0.3s for the step function going from 0 to 1) then use an if statement)

V=if(time&lt;0.3,0,5*exp(-12.5*(time-0.3)))