The circuit that I am simulating is a Vishay PTC thermistor model number PTCTL7MR100SBE.
I have a timestep too small error when trying to simulate.
I tried running the PTC through a pulse voltage of 500V with a rise time of 0.4us and a fall time of 1ps. The error seems to be happening at the fall time.
Can anybody advise me on how to solve the error?
As already stated in the comments the problems seems to lie in the fall time of your supply. This can sometimes be overcome by either changing the solver or checking whether your simulation makes sense at all as described in this question.
After tweaking the simulation a bit, I manage to make it run by slightly increasing the fall time from 1ps to 100ps and reducing the maximum stepsize to 0.1ps.
Like mentioned in the comments, your source is too ideal and creates a very extreme and impractical \$\frac{dV}{dt}\$. That ideal triangle shape you're simulating will also never show up in real life. I've done DO-160 Lighting Tests, and their waveform specifications always have some kind of RC smoothing on them to be more realistic.
I was able to get your simulation to simulate using a slightly less intense waveform via an RC and also forcing the maximum time step size to 1p (see the .tran statement). But keep in mind this takes a long time to simulate.
Since you didn't explain what you're trying to measure, I have to assume the following might be important too. If you need to avoid current limiting via loading from that series resistor, then you can pass the smoothed waveform through a Gain=1 voltage buffer like so:
