Perfect component on LTspice
LTspice solves nonlinear differential equations numerically. It's not an AI system that can figure out intelligently whether a diode is conducting or not, and thus solve your homework questions or whatever. 

In general I think that models that are not "smooth" will cause convergence problems. 

So a perfect resistor is fine, but a perfect diode would have to be approximated to the desired degree of accuracy by something like a arctangent function such that the diode current vs. voltage does not have an abrupt change. 

A perfect capacitor or inductor would cause a singularity if you connected it to a voltage source or a current source respectively that changed instantly. 

LTSpice tries to prevent you from making this mistake by helpfully ignoring you when you set rise and fall times to zero and using default values that are rather large. Similarly, the "switches" in LTspice and PSpice are made imperfect.