Molecular complexes and molecular clusters represent a broad class of systems with interesting chemical and physical properties. Such systems can be naturally partitioned into fragments each representing a molecule or several molecules. Q-Chem contains a set of methods designed to use such partitioning either for physical or computational advantage. Some of these methods were developed and implemented by Dr. Rustam Z. Khaliullin at the University of California–Berkeley, with Profs. Martin Head-Gordon and Alexis Bell; the open shell versions were developed by Paul Horn at Berkeley, working with Martin Head-Gordon. Others were developed by Dr. Leif D. Jacobson and Ka Un Lao working with Prof. John M. Herbert at Ohio State University.

The list of methods that use partitioning includes:

Initial guess at the MOs as a superposition of the converged MOs on the isolated fragments (FRAGMO guess) [683].

Constrained (locally-projected) SCF methods for molecular interactions (SCF MI methods) between closed shell fragments [683], and also open shell fragments [684].

Single Roothaan-step (RS) correction methods that improve FRAGMO and SCF MI description of molecular systems [683, 684].

Automated calculation of the BSSE with counterpoise correction method (full SCF and RS implementation).

Energy decomposition analysis and charge transfer analysis [685, 686, 684].

Analysis of intermolecular bonding in terms of complementary occupied-virtual pairs [686, 687, 684].

The variational explicit polarization (XPol) method, a self-consistent, charge-embedded, monomer-based SCF calculation [688, 689, 456]

Symmetry-adapted perturbation theory (SAPT), a monomer-based method for computing intermolecular interaction energies and decomposing them into physically-meaningful components [690, 691].

XPol+SAPT (XSAPT), which extends the SAPT methodology to systems consisting of more than two monomers [689, 456, 692].

XSAPT(KS)+D, a dispersion-corrected version of XSAPT that affords accurate intermolecular interaction energies at very low cost [693, 694, 695].

The electrostatically embedded many-body expansion [696] and the fragment molecular orbital method [697, 698], for decomposing large clusters into small numbers of monomers, facilitating larger calculations.

Another fragment-based approach, the Effective Fragment Potential (EFP) method [665], was developed by Prof. Lyudmila V. Slipchenko at Purdue University and Prof. Anna I. Krylov at USC; this method is described in Section 11.5.