How to define a PSD ?

Hello, I'm trying to run a random response analyse. I want to define a PSD spectrum in Abaqus and i don't know what should i put in the PSD table (gravity base motion). I only have the RMS of my PSD spectrum and for each frequency the amplitude in term of acceleration (ie : from 20 to 100Hz i have 0.0044g2/Hz; from 100 to 300Hz ... ). However Abaqus wants me to fill a real and a imaginary part to define the spectrum. Unfortunately, I didn't find anything about that on the Abaqus User's Guide so i really don't know how to procede. Should i only fill the real part ? Your help will be greatly appreciated and by the way, I appologize for my english. Clement