AbstractPseudopotential theory is introduced into the ab initio FSGO molecular fragment method. Pseudopotential molecular fragments using pseudopotentials of double‐zeta quality are characterized, and a method of their assembly into larger molecules is presented. Core–valence electron separation is achieved at both levels of the molecular calculations. Heteroatom incorporation into the method is also considered. Applications of the pseudo‐FSGO molecular fragment method to hydrocarbons and simple molecular systems containing heteroatoms are discussed. Results are compared to those of the original FSGO method and experiment.