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, Yuezhi Mao, Dr. Jonathan Thirman, Dr. Daniel S. Levine, and Qinghui Ge 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).[Khaliullin et al.(2006)Khaliullin, Head-Gordon, and Bell]
Constrained (locally-projected) SCF methods for molecular interactions (SCF MI methods) between both closed-shell[Khaliullin et al.(2006)Khaliullin, Head-Gordon, and Bell] and open-shell[Horn et al.(2013)Horn, Sundstrom, Baker, and Head-Gordon] fragments.
Single Roothaan-step (RS) correction methods that improve FRAGMO and SCF MI description of molecular systems.[Khaliullin et al.(2006)Khaliullin, Head-Gordon, and Bell, Horn et al.(2013)Horn, Sundstrom, Baker, and Head-Gordon]
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,[Khaliullin et al.(2007)Khaliullin, Cobar, Lochan, Bell, and Head-Gordon, Khaliullin et al.(2008)Khaliullin, Bell, and Head-Gordon, Horn et al.(2013)Horn, Sundstrom, Baker, and Head-Gordon] and the analysis of intermolecular bonding in terms of complementary occupied-virtual pairs (COVPs).[Khaliullin et al.(2008)Khaliullin, Bell, and Head-Gordon, Khaliullin et al.(2009)Khaliullin, Bell, and Head-Gordon, Horn et al.(2013)Horn, Sundstrom, Baker, and Head-Gordon]
An improved version (“second generation”) of the ALMO-EDA method,[Horn and Head-Gordon(2015), Horn and Head-Gordon(2016), Horn et al.(2016a)Horn, Mao, and Head-Gordon, Horn et al.(2016b)Horn, Mao, and Head-Gordon], including its extension to single-bond interactions.[Levine et al.(2016)Levine, Horn, Mao, and Head-Gordon]
An extension of the ALMO-EDA to MP2.[Thirman and Head-Gordon(2015), Thirman and Head-Gordon(2017)]
The adiabatic ALMO-EDA method that bridges intermolecular interactions and molecular properties.[Mao et al.(2017)Mao, Horn, and Head-Gordon]
The variational explicit polarization (XPol) method, a self-consistent, charge-embedded, monomer-based SCF calculation.[Xie et al.(2008)Xie, Song, Truhlar, and Gao, Jacobson and Herbert(2011), Herbert et al.(2012)Herbert, Jacobson, Lao, and Rohrdanz]
Symmetry-adapted perturbation theory (SAPT), a monomer-based method for computing intermolecular interaction energies and decomposing them into physically-meaningful components.[Jeziorski et al.(1994)Jeziorski, Moszynski, and Szalewicz, Szalewicz(2012)]
XPol+SAPT (XSAPT), which extends the SAPT methodology to systems consisting of more than two monomers.[Jacobson and Herbert(2011), Herbert et al.(2012)Herbert, Jacobson, Lao, and Rohrdanz, Jacobson et al.(2013)Jacobson, Richard, Lao, and Herbert]
Closed- and open-shell AO-XSAPT(KS)+D, a dispersion-corrected version of XSAPT in atomic orbital basis that affords accurate intermolecular interaction energies at very low cost.[Lao and Herbert(2012b), Lao and Herbert(2013), Lao and Herbert(2015)]
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.[Lao and Herbert(2016)]
The electrostatically-embedded many-body expansion[Dahlke and Truhlar(2007), Richard et al.(2014a)Richard, Lao, and Herbert, Richard et al.(2014b)Richard, Lao, and Herbert, Lao et al.(2016)Lao, Liu, Richard, and Herbert] and the fragment molecular orbital method,[Kitaura et al.(1999)Kitaura, Ikeo, Asada, Nakano, and Uebayasi, Fedorov and Kitaura(2007)] for decomposing large clusters into small numbers of monomers, facilitating larger calculations.
The Ab Initio Frenkel Davydov Model,[Morrison et al.(2014)Morrison, You, and Herbert, Morrison and Herbert(2015)] a low-order scaling, highly parallelizable approach to computing excited state properties of liquids, crystals, and aggregates.
TDDFT for molecular interactions [TDDFT(MI)], an excited-state extension of SCF MI that offers a reduced-cost way to compute excited states in molecular clusters, crystals, and aggregates.[Liu and Herbert(2015), Liu and Herbert(2016), Herbert et al.(2016)Herbert, Zhang, Morrison, and Liu]
Another fragment-based approach, the Effective Fragment Potential (EFP) method,[Ghosh et al.(2010)Ghosh, Kosenkov, Vanovschi, Williams, Herbert, Gordon, Schmidt, Slipchenko, and Krylov] was developed by Prof. Lyudmila Slipchenko at Purdue University and Prof. Anna Krylov at USC; this method is described in Section 12.5.