The next example is contained in the files MC_2_control.sp and MC_2_circ.sp from folder /examples/Monte_Carlo/. MC_2_control.sp is a ngspice script (see 17.8). It starts a loop by setting the random number generator seed value to the value of the loop counter, sources the circuit file MC_2_circ.sp, runs the simulation, stores a raw file, makes an fft, saves the oscillator frequency thus measured, deletes all outputs, increases the loop counter and restarts the loop. The netlist file MC_2_circ.sp contains the circuit, which is the same ring oscillator as of example 2. However, now the MOS model parameter set, which is included with this netlist file, inherits some AGAUSS functions (see 2.8.5) to vary threshold voltage, mobility and gate oxide thickness of the NMOS and PMOS transistors. This is an approach similar to what commercial foundries deliver within their device libraries. So this example may be your source for running Monte Carlo with commercial libs. Start example 3 by calling

ngspice -o MC_2_control.log MC_2_control.sp