I am working with a function in Mathematica and I am getting some inconsistencies when I plot it. As I really need to understand were this comes from I would appreciate any help. 

I am working with a function of the following form:

`f2b[b_] = 
 Exp[-1000 - 2 Sqrt[1000*b] - b]*(Erfi[Sqrt[1000]] - 
    Erfi[Sqrt[1000] + Sqrt[b]])`

From my understanding this function should go to zero for very small b because both parts, the Exp function as well as the difference between the imaginary error functions, should vanish.

Mathematica calculates:

`f2b[E^-30] // N = -3.4517046881*10^-7` as expected. 

BUT if I plot this function:

`Plot[{f2b[E^b], 0}, {b, -50, 0}, 
 PlotStyle -> {{Blue, Thick}, {Red, Thick}}]`

Mathematica shows me a plot which is approaching a value of about -0.0175 for small b and has therefore an absolute value much larger the the value at E^-30 calculated above.

If I tabulate, interpolate and plot the function:

`t = Table[{b, f2b[E^b] // N}, {b, -50, 0}];
f = Interpolation[t];
Plot[{f[b], 0}, {b, -50, 0}, 
 PlotStyle -> {{Blue, Thick}, {Red, Thick}}]`

The plot is looking much different and is approaching zero as expected.

I am wondering whether this has to do something with overflow problems in graphics but I am not sure. 

Thanks in advance.

Silky