For a description of SAPT see: B. Jeziorski, R. Moszynski, and K. Szalewicz “Perturbation Theory Approach to Intermolecular Potential Energy Surfaces of van der Waals Complexes”, Chem. Rev. 94, 1887-1930 (1994). This review article contains references to the original papers where the SAPT methods were developed. The formulas used in the program are gathered in a paper by B. Jeziorski, R. Moszynski, A. Ratkiewicz, S. Rybak, K. Szalewicz, and H.L. Williams published in Methods and Techniques in Computational Chemistry: METECC-94, edited by E. Clementi, STEF, Cagliari, 1993, Vol. B, p. 79.
SAPT(DFT) method is described in: A. J. Misquitta, R. Podeszwa, B. Jeziorski, and K. Szalewicz, “Intermolecular potentials based on symmetry-adapted perturbation theory including dispersion energies from time-dependent density functional calculations”, J. Chem. Phys., 123, 214103 (2005).
A fairly extensive SAPT2020 User Guide is available in pdf format.
The autoPES manual is avaiable here.