How to understand the Convergence and Error

Hi, guys. My programme cannot get convergence, and I found something eerie which makes me crying out... Here's my programme.

clear all;
close all;
a=10000;
b=1/a;
c=40000000;
d=2.33798773196774;
tr=1e-7;
t0=0.316398751051366;
per0=8.854187817e-12;
tp=(b-2*tr)/2;
Q=2*per0*d*c^2*(1+2*t0/tr*exp(-tr/t0)+2*(t0/tr)^2*exp(-tr/t0)-2*(t0/tr)^2+...
t0/tr*(1-exp(-tr/t0))*(1-exp(-tp/t0)));

When t0 is around 0.316398751055, I got a negtive value but not I expected in theoretical calculation.

For instance, when t0=0.31639875103, Q=10.446538438097960,

but when t0=0.31639875105, Q=-248.3157427204564,

and when t0=0.31639875106, Q=10.446538434426026. I cannot understand that. Thx for your kind help.