ngspice/README.optran

Adding transient simulation top generate the operating point of a circuit

The operating point of a circuit has to be established before a transient, ac, 
or noise simulation may be started.

The procedure, as found in cktop.c is as follows:
1) Iterate the matrix with the starting values
If this fails:
2) Start gmin stepping
If this fails:
3) start source stepping

If this again fails, bail out.

In this branch a fourth method is added:
4) Start a transient simulation for a specific amount of time.
   Keep all data local. Keep the matrix values as an
   operating point for the simulation following.
If this fails, finally stop.

The time for transient is determined a priori. If a transient
simulation follows, choose 100 time the TSTEP value. If an ac
or noise simulation follows, take the inverted starting
frequency divided by 10 as the optran simulation time.

A method to stop the op transient simulation, when all nodes
have become stable, might be desirable, but has not yet been implemented.

Step 1 may be skipped by
option noopiter

Step 2 may be skipped by
option gminsteps=0

Step 3 may be skipped by 
option srcsteps=0

The transient op calculation uses the 'UIC' flag for its startup.
Basically two applications come to mind: You may use this with
constant voltage and current sources and simulate until the 
output is stable. A method to automatically stop the op transient
simulation when all nodes have become stable, might be desirable,
but has not yet been implemented.
One also may use the optran for a first transient simulation, maybe
with different step sizes, before the next (tran, ac, pz, noise)
simulation is commencing.