Symmetry-adapted perturbation theory (SAPT) is a method for calculations of intermolecular (noncovalent) interaction energies. The set of SAPT codes that is described here, the current version named SAPT2020, includes virtually all variants of SAPT developed so far, among them two-body SAPT based on perturbative, coupled cluster, and density functional theory descriptions of monomers, three-body SAPT, and two-body SAPT for some classes of open-shell monomers. The properties of systems governed by noncovalent interactions can be predicted only if potential energy surfaces (force fields) are available. SAPT is the preferred approach for generating such surfaces since it is seamlessly connected to the asymptotic expansion of interaction energy. SAPT2020 includes codes for automatic development of such surfaces, enabling generation of complete dimer surfaces with a rigid monomer approximation for dimers containing about one hundred atoms. These codes can also be used to obtain surfaces including internal degrees of freedom of monomers.

The Hartree−Fock plus dispersion plus first-order correlation (HFDc (1) ) method consists in augmenting the HF interaction energy by the correlation part of the firstorder interaction energy and the second-order dispersion and exchange-dispersion energies. All of the augmentation terms are computed using the symmetry-adapted perturbation theory based on density functional theory description of monomers [SAPT(DFT)]; thus, HFDc (1) is a fully ab initio method. A partly empirical version of this method, HFD as c (1) , uses a damped asymptotic expansion for the dispersion plus exchange-dispersion term fitted to SAPT(DFT) ab initio values. The HFDc (1) interaction energies for dimers in the S22, S66, S66x8, NCCE31, IonHB, and UD-ARL benchmark data sets are more accurate than those given by most ab initio methods with comparable costs. HFDc (1) can be used routinely for dimers with nearly 200 atoms, such as included in the S12L benchmark set, giving results comparable to those obtained by the most expensive methods applicable.

This work studies the performance of doubly occupied configuration interaction treatments based on the determination of wave functions and on the direct variational evaluation of two‐electron reduced density matrices in both, restricted and unrestricted approaches. We describe potential energy curves and spin contamination curves predicted by these methods for N‐electron molecular systems in their electronic ground states, at internuclear distances lying in intervals from near equilibrium geometries to dissociation limits. The corresponding numerical determinations in terms of energies, expectation values of the N‐electron spin‐squared operator, local spins, and electronic populations of the dissociation products are compared with those provided by the full configuration interaction procedure. An analysis of these results allows us to assess the abilities of the restricted and unrestricted methodologies and to illuminate the advantages and shortcomings of each of these treatments.

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