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WIREs Comput Mol Sci
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Q‐Chem: an engine for innovation

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Abstract Q‐Chem is a general‐purpose electronic structure package featuring a variety of established and new methods implemented using innovative algorithms that enable fast calculations of large systems on regular laboratory workstations using density functional and wave‐function‐based approaches. It features an integrated graphical interface and input generator, a large selection of functionals and correlation approaches including methods for electronically excited states and open‐shell systems. In addition to serving the computational chemistry community, Q‐Chem also provides an excellent development platform. © 2012 John Wiley & Sons, Ltd. This article is categorized under: Software > Quantum Chemistry

A snapshot of the code submitted by John Pople to Q‐Chem in August 2003. Written elegantly in C++, it significantly improves the efficiency of the original Head–Gordon–Pople integral code in Q‐Chem and speeds up Hartree–Fock and hybrid density functional theory calculations.

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A subroutine updating T2 amplitudes for CCD. The contractions and other tensor operations are coded using a convenient LaTeX‐style programming interface. Because low‐level details are hidden, such routines are very portable, easy to read and modify. The effort required to implement new models is also significantly reduced by the interface.

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Character of excited states in model systems representing green fluorescent protein (GFP) has been assigned by using attachment–detachment density analysis.66 In particular, the attachment density allowed identification of excited states as local excitations (localized on the chromophore), or as charge‐transfer‐to‐solvent (CTTS) ones, in which an electron is excited from the chromophore's molecular orbitals into a nearby cavity. Shown are excitation energies of a CTTS‐like state [SOS‐CIS(D)/6‐31(2+,+)G(d,p)] and the respective attachment densities in different quantum mechanics clusters mimicking the GFP active site.66

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Absorption spectra of para‐nitroaniline in water, dioxane, and cyclohexane. The π → π* state has strong charge‐transfer character and large dipole moment resulting in red solvatochromatic shift in polar solvents. Accounting for solvent polarization in response to electronic excitation58 or ionization60 is necessary for quantitative agreement with experiment.

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Quantum Mechanics/Molecular Mechanics (QM/MM) calculations of optical and redox properties of the green fluorescent protein have advanced our understanding of its photophysics.55 The chromophore and neighboring residues are included in the QM part, which can be described by DFT, SOS‐CIS(D), or EOM‐CCSD, whereas the rest of the protein and solvent is described by a force field.

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Q‐Chem includes a variety of equation‐of‐motion coupled‐cluster (EOM‐CC) methods enabling accurate calculations of electronically excited and open‐shell species. In the EOM models for electronically excited states (EOM‐EE, top), the reference is the closed‐shell ground‐state Hartree–Fock determinant (Φ0) and the operator conserves the number of α and β electrons generating a set of excited determinants (e.g., {,} in EOM‐CCSD). Nonparticle conserving and spin‐flipping operators of EOM‐IP/EA/SF open a route to the multiconfigurational wave functions encountered in radicals, diradicals, triradicals, and bond‐breaking processes.

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Thousands of Q‐Chem copies are being used, as estimated from the number of issued licenses. The citations of the two Q‐Chem papers1,2 has reached 200 per year in 2010, and is growing (as reported by ISI Web of Science). As part of the IBM World Community Grid, about 350,000 Q‐Chem calculations are performed every day by the Harvard Clean Energy Project,11 which is powered by Q‐Chem, free of charge.

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Independent comparison of an established generalized gradient approximation (BLYP) against an established hybrid (B3LYP), a recent range‐separated hybrid (ωB97X), and a range‐separated hybrid that includes an empirical long‐range dispersion correction (ωB97X‐D). MAE (mean average errors) are computed for the atomization energies (48 reactions comprising the G3/05 test set) and weak interactions (25 intermolecular complex binding energies).23,24

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