The following design rules must be observed for the simulation to run correctly. Note that most circuits obey them anyway and they do not impose serious limitations on the capability of the simulator.
There must always be at least one ground symbol on every circuit.
Every node on the circuit must have a dc path to ground. For example, two capacitors in series form a node which does not have DC path to ground. If you do have a floating node, you can provide a DC path to ground by connecting a high value resistor (e.g. 1G) between it and ground. Capacitors without initial conditions do not have a DC path. But if you set an initial condition on a capacitor a DC path is formed and this method is an alternative to using a resistor to provide a DC path.
Also note that inductors with an initial condition do not have a DC path. This is because they are treated as a constant current during the calculation of the DC bias point.
If using a high value resistor to make a DC path for a transformer winding, we recommend that you also place a low value capacitor in parallel with it. This is not always necessary but can help avoid problems with transient analysis convergence. It is best to choose a realistic value that would represent what the capacitance would really be in the real-world circuit.
There must not be any zero resistance loops constructed from voltage sources, inductors without initial conditions or capacitors with initial conditions. If you do have such loops you can insert a low value resistor. You should choose a value that represents what the resistance would be in the real world, e.g. the actual winding resistance of an inductor, and never use an unrealistically small value. Very small resistances (e.g. 1 fempto-Ohm) can cause convergence problems.
For loops containing inductors you can break the loop by adding an initial condition to the inductor instead of adding a resistor.
Failure to observe the above usually leads to a Singular Matrix