RE: include a opamp LM741 model in gnucap

Two things:

1) you can include another file with the .include command

2) You should name your opamp X_opamp (X for a subcircuit call)
instead of U_opamp

So you file will look like this:

* Spice netlister for gnetlist
.INCLUDE LM741.cir
R2 2 6 100K
R1 a 2 1K
X_opamp(0 2 1 4 6)LM741
C1 2 6 100u
V_neg 4 0 -12V
V_pos 1 0 12V
.END

Note that there is no default path for .INCLUDE commands.  You must
specify the full path, relative, or absolute.
-Alan