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 (e.g. the ALMO-EDA method and its most recent updates/extensions) were developed and implemented by Dr. Rustam Z. Khaliullin, Dr. Paul R. Horn, Dr. Yuezhi Mao, Dr. Jonathan Thirman, Dr. Daniel S. Levine, Dr. Qinghui Ge, and Matthias Loipersberger working with Prof. Martin Head-Gordon at the University of California–Berkeley. Other methods [e.g., the XSAPT family of methods and TDDFT(MI)] were developed by Drs. Leif Jacobson, Ka Un Lao, and Jie Liu working with Prof. John 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).473
Automated calculation of the BSSE with counterpoise correction method (full SCF and RS implementation).
The original version the ALMO-EDA method (energy decomposition analysis based on absolutely localized molecular orbitals), including the associated charge transfer analysis,472, 470, 401 and the analysis of intermolecular bonding in terms of complementary occupied-virtual pairs (COVPs).470, 471, 401
A stable and physically-motivated energy decomposition approach, SAPT/cDFT, in which cDFT is used to define the charge-transfer component of the interaction energy and SAPT defines the electrostatic, polarization, Pauli repulsion, and van der Waals contributions.540