[Help-gnucap] Minimal stepsize in trasient analysis

[smkarim]

In transient analysis, can the stepsize be smaller than 1e-12?
I'm not succeeding doing so...
For example:
.gen Delay=0.5e-10 width=0.5e-10 period=1e-10 max=1 min=0 rise=0 fall=0
.tran 0 1e-10 1e-12 

If I write 0.5e-12 instead of the last 1e-12 then I get the message "very
backward time step".

Can anyone explain me this? I nees more accurate transient analysis...