what ac model PSpice uses for a diode in reverse bias?

the model comprises of a capacitance and a resistor. i know that much, but what is the values for the R and C.

I have some formulas for R and C from the book "Semiconductor Device Modeling with SPICE"

R is equal to the slope of the I-V curve for the diode at its operating point.

and C is equal to C_junction + C_diffusion
there are formulas for each C.

I use those formulas to simulate a simple circuit myself and then I compare my results with SPICE results.

results are consistent for direct bias and also for breakdown region, but I get strange results in SPICE in reverse bias.

does anyone know what ac model SPICE uses for a diode in reverse bias?