Exponential integral function

Hi,

I'm trying to use expint matlab's function for exponential integral to solve something like this:

pc= a*(expint((-(r^2))/(4*k*(tm-t0)))- expint((-(r^2))/(4*k*tm)))

in this problem I need to evaluate the exponential integral for x<0, and I wonder if I can actually do that with the expint function or if that function is only defined for x>0. I got strange values when I employed it in my calculations.

I was looking for a solution and I found a function for exponential integral named Ei within the mfunlist (Special Functions of Applied Mathematics). So I wonder if that function does work for x<0 and therefore is the one I should use.

Thanks.