Advances in theory and algorithms for electronic structure calculations must be incorporated into program packages to enable them to become routinely used by the broader chemical community. This work reviews advances made over the past five years or so that constitute the major improvements contained in a new release of the Q-Chem quantum chemistry package, together with illustrative timings and applications. Specific developments discussed include fast methods for density functional theory calculations, linear scaling evaluation of energies, NMR chemical shifts and electric properties, fast auxiliary basis function methods for correlated energies and gradients, equation-of-motion coupled cluster methods for ground and excited states, geminal wavefunctions, embedding methods and techniques for exploring potential energy surfaces.

Q-Chem 2.0 is a new release of an electronic structure program package, capable of performing first principles calculations on the ground and excited states of molecules using both density functional theory and wave function-based methods. A review of the technical features contained within Q-Chem 2.0 is presented. This article contains brief descriptive discussions of the key physical features of all new algorithms and theoretical models, together with sample calculations that illustrate their performance.

We apply tensor methods to formulate theories of electron correlation in nonorthogonal basis sets. The resulting equations are manifestly invariant to nonorthogonal basis transformations, between functions spanning either the occupied or virtual subspaces of the one-particle Hilbert space. The tensor approach is readily employed in either first or second quantization. As examples, second-order Mo/ller–Plesset perturbation theory, and coupled cluster theory with single and double substitutions, including noniterative triples, are recast using the tensor formalism. This gives equations which are invariant to larger classes of transformations than existing expressions. Procedures for truncating these equations are discussed.

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Articles you may be interested in NMR shielding tensors for density fitted local second-order Møller-Plesset perturbation theory using gauge including atomic orbitals J. Chem. Phys. 137, 084107 (2012); 10.1063/1.4744102Orbital-dependent correlation energy in density-functional theory based on a second-order perturbation approach: Success and failure Explicitly correlated second-order Møller-Plesset methods with auxiliary basis sets Second order Mo/ller-Plesset perturbation theory without basis set superposition error A new ansatz for local electron correlation is introduced, which truncates double substitutions subject to a triatomics in molecules ͑TRIM͒ criterion. TRIM includes all double substitutions in which one occupied-virtual substitution is atomic while the other substitution can be nonlocal ͑a cubic number, before cutoffs͒. With an additional approximation, the TRIM second-order Mo "llerPlesset perturbation theory ͑MP2͒ model can be noniteratively solved; this is the model that is implemented. Results are shown for absolute energies of alkane and polyene chains, rotational barriers of substituted ethylenes and benzenes, and association energies of the water and neon dimers. Over 99.7% of the untruncated MP2 energy is recovered for the test cases, and the relative energies of small systems are in error by less than 0.1 kcal/mol. By contrast, a diatomics in molecules ͑DIM͒ truncation recovers about 95% of the full MP2 energy, and yields errors several times larger for relative energies.