gEDA-user: circuit simulation - backend library?

[kitts]

Hi All,

The information i seek does not likely belong to thins list but i believe 
here could have the knowledge...

I am looking for a good library/tool to be used for circuit simulation. My 
requirement from this tools are:

1) Ability to run the simulation for a given duration, say like 10 seconds 
of simulation. And the ability to 'pause' the simulation and read 
instantaneous values at various points.

2) Ability to change the circuit while simulating or the ability to start a 
new simulation by providing an initial instantaneous state of the various 
points in a circuit.

I am looking for backend tool so one can build a GUI over it.

Your help is much appreciated. Thanks.
-- 
Cheers!
kitts