I have a book on Gaussian but it mainly just explains how to use the program, it only gives very brief explanations of the theory behind how Gaussian does what it does. I want to gain an understanding of how Gaussian performs the various types of calculations that it can do. I understand the concept of the potential energy surface but I'm trying to figure out how it uses the potential energy surface to calculate various things about the molecule such as IR vibrations, zero point energy, thermochemical properties, UV-Vis spectra etc. Right now I'm trying to get my head around frequency scans and how Gaussian uses the potential energy surface to calculate IR frequencies + their intensities and other things like zero-point energy and thermochemical properties etc. I've been trying to figure it out in my head because I can't find any explanations. By visualising the PES of carbon monoxide, I can see how Gaussian can use it to calculate the displacement of a particular vibration because the bond length is represented on the x-axis. I can see how it can calculate IR vibrations if the energies of the normal modes of vibration are known but I can't figure out how it could calculate the energies of the normal modes as well as the zero-point energy from the PES. Anyone know of a book or website that explains in detail how Gaussian does what it does?