Qucs-S is a spin-off of the Qucs circuit simulator. The "S" letter
indicates SPICE. It merges the power of SPICE and the simplicity of
the Qucs GUI. Qucs-S is not a simulator by itself, but it requires
to use a simulation backend with it. The schematic document format of
Qucs and Qucs-S are fully compatible. Qucs-S allows to use the following
simulation kernels:

Ngspice(recommended optional dependency), XYCE and SpiceOpus.