The function cubicnl in the Faust software distribution (file effect.lib) implements cubic nonlinearity distortion . The Faust programming example cubic_distortion.dsp provides a real-time demo with adjustable parameters, including an offset for bringing in even harmonics. The free, open-source guitarix application (for Linux platforms) uses this type of distortion effect along with many other guitar effects.
Passive String Terminations
Enhancing Even Harmonics