Nathan J. DeYonker

The correlation consistent composite approach (ccCA): An alternative to the Gaussian-n methods

An alternative to the Gaussian-n (G1, G2, and G3) composite methods of computing molecular energies is proposed and is named the correlation consistent composite approach (ccCA, ccCA-CBS-1, ccCA-CBS-2). This approach uses the correlation consistent polarized valence (cc-pVXZ) basis sets. The G2-1 test set of 48 enthalpies of formation (DeltaHf), 38 adiabatic ionization potentials (IPs), 25 adiabatic electron affinities (EAs), and 8 adiabatic proton affinities (PAs) are computed using this approach, as well as the DeltaHf values of 30 more systems. Equilibrium molecular geometries and vibrational frequencies are obtained using B3LYP density functional theory. When applying the ccCA-CBS method with the cc-pVXZ series of basis sets augmented with diffuse functions, mean absolute deviations within the G2-1 test set compared to experiment are 1.33 kcal mol(-1) for DeltaHf,0.81 kcal mol(-1) for IPs, 1.02 kcal mol(-1) for EAs, and 1.51 kcal mol(-1) for PAs, without including the high-level correction (HLC) contained in the original Gn methods. Whereas the HLC originated in the Gaussian-1 method as an isogyric correction, it evolved into a fitted parameter that minimized the error of the composite methods, eliminating its physical meaning. Recomputing the G1 and G3 enthalpies of formation without the HLC reveals a systematic trend where most DeltaHf values are significantly higher than experimental values. By extrapolating electronic energies to the complete basis set (CBS) limit and adding G3-like corrections for the core-valence and infinite-order electron correlation effects, ccCA-CBS-2 often underestimates the experimental DeltaHf, especially for larger systems. This is desired as inclusion of relativistic and atomic spin-orbit effects subsequently improves theoretical DeltaHf values to give a 0.81 kcal mol(-1) mean absolute deviation with ccCA-CBS-2. The ccCA-CBS method is a viable black box method that can be used on systems with at least 10-15 heavy atoms.

The correlation-consistent composite approach: Application to the G3/99 test set

The correlation-consistent composite approach (ccCA), an ab initio composite technique for computing atomic and molecular energies, recently has been shown to successfully reproduce experimental data for a number of systems. The ccCA is applied to the G3/99 test set, which includes 223 enthalpies of formation, 88 adiabatic ionization potentials, 58 adiabatic electron affinities, and 8 adiabatic proton affinities. Improvements on the original ccCA formalism include replacing the small basis set quadratic configuration interaction computation with a coupled cluster computation, employing a correction for scalar relativistic effects, utilizing the tight-d forms of the second-row correlation-consistent basis sets, and revisiting the basis set chosen for geometry optimization. With two types of complete basis set extrapolation of MP2 energies, ccCA results in an almost zero mean deviation for the G3/99 set (with a best value of -0.10 kcal mol(-1)), and a 0.96 kcal mol(-1) mean absolute deviation, which is equivalent to the accuracy of the G3X model chemistry. There are no optimized or empirical parameters included in the computation of ccCA energies. Except for a few systems to be discussed, ccCA performs as well as or better than Gn methods for most systems containing first-row atoms, while for systems containing second-row atoms, ccCA is an improvement over Gn model chemistries.

Accurate thermochemistry for transition metal complexes from first-principles calculations

The correlation consistent Composite Approach or ccCA is an ab initio model chemistry based on the single reference MP2 level of theory. By adjusting the basis set and level of theory of the core valence additive correction, ccCA is capable of reliable thermochemical predictions of inorganic and organometallic transition metal-containing molecules, as well as achieving chemical accuracy on main group species, with a mean absolute deviation of 0.89 kcal mol(-1) against the 147 enthalpies of formation in the G2/97 test set. For a set of 52 complexes containing elements Sc-Zn, ranging in size from diatomics to Ni(PF(3))(4) and Fe(C(5)H(2))(2), ccCA on average predicts enthalpies of formation to within +/-3 kcal mol(-1) of the experimental result with a mean absolute deviation of 2.85 kcal mol(-1) and a root mean square deviation of 3.77 kcal mol(-1). The ccCA methodology is a significant step toward quantitative theoretical modeling of transition metal thermodynamics.

Towards the intrinsic error of the correlation consistent Composite Approach (ccCA)

The correlation consistent Composite Approach (ccCA) has been made more robust by (a) modifying the basis set used in computing B3LYP equilibrium geometries and harmonic vibrational frequencies so that the correlation consistent basis sets are used throughout ccCA; (b) separately extrapolating the MP2 and Hartree–Fock complete basis set limit energies; (c) uniformly treating unrestricted open shell wave functions; (d) utilizing newly recommended enthalpies of formation for C, B, Al, and Si atoms; and (e) using theoretically derived vibrational scale factors. This modified ccCA formulation has been used to compute the 454 energetic properties (enthalpies of formation, dissociation energies, ionization potentials, electron affinities, and proton affinities) in the G3/05 test set. This new formulation, which does not contain any optimized parameters, has a small systematic statistical bias (mean signed deviation of −0.20 kcal mol−1), and has a mean absolute deviation of 1.01 kcal mol−1 with the incorporation of modification d) or 0.99 kcal mol−1 without. This is compared to a G4(MP2) MAD of 1.04 kcal mol−1 and a G3(MP2) MAD of 1.39 kcal mol−1. These modifications result in minimal change with respect to the computational requirements of the current ccCA methodology. The ccCA model chemistry is the first MP2-based model chemistry to achieve an accuracy of ± 1.00 kcal mol−1 for the G3/05 training set without any optimized parameters, and it is the only MP2-based model chemistry uniformly applicable to systems comprised of elements from H to Kr.

Performance of the correlation consistent composite approach for transition states: A comparison to G3B theory

The correlation consistent composite approach (ccCA) was applied to the prediction of reaction barrier heights (i.e., transition state energy relative to reactants and products) for a standard benchmark set of reactions comprised of both hydrogen transfer reactions and nonhydrogen transfer reactions (i.e., heavy-atom transfer, SN2, and unimolecular reactions). The ccCA method was compared against G3B for the same set of reactions. Error metrics indicate that ccCA achieves chemical accuracy with a mean unsigned error (MUE) of 0.89 kcal/mol with respect to the benchmark data for barrier heights; G3B has a mean unsigned error of 1.94 kcal/mol. Further, the greater accuracy of ccCA for predicted reaction barriers is compared to other benchmarked literature methods, including density functional (BB1K, MUE=1.16 kcal/mol) and wavefunction-based [QCISD(T), MUE=1.10 kcal/mol] methods.

Hartree-Fock complete basis set limit properties for transition metal diatomics.

Numerical Hartree-Fock (HF) energies accurate to at least 1 microhartree are reported for 27 diatomic transition-metal-containing species. The convergence of HF energies toward this numerical limit upon increasing the basis set size has been investigated, where standard nonrelativistic all-electron correlation consistent basis sets and augmented basis sets, developed by Balabanov and Peterson [J. Chem. Phys. 123, 064107 (2005)], were employed. Several schemes which enable the complete basis set (CBS) limit to be determined have been investigated, and the resulting energies have been compared to the numerical Hartree-Fock energies. When comparing basis set extrapolation schemes, those in the form of exponential functions perform well for our test set, with mean absolute deviations from numerical HF energies of 234 and 153 microE(h), when the CBS limit has been determined using a two-point fit as proposed by Halkier et al. [Chem. Phys. Lett. 302, 437 (1999)] on calculations of triple- and quadruple-zeta basis set qualities and calculations of quadruple- and quintuple-zeta basis set qualities, respectively. Overall, extrapolation schemes in the form of a power series are not recommended for the extrapolation of transition metal HF energies. The impact of basis set superposition error has also been examined.

The Correlation Consistent Composite Approach (ccCA): Efficient and Pan-Periodic Kinetics and Thermodynamics

The correlation consistent Composite Approach (ccCA) methodology is the most accurate MP2-based model chemistry developed to date and contains no optimized empirically based parameters. As the method has continued to evolve and found new areas of application, we provide in this chapter a historical context of the developments and discoveries, an overview of variations of the ccCA model chemistry, and some discourse on future research thrusts. The success of ccCA for chemical applications is reviewed.

Prediction of hydrocarbon enthalpies of formation by various thermochemical schemes

The correlation consistent composite approach (ccCA) has been used to compute the enthalpies of formation (ΔHf′s) for 60 closed‐shell, neutral hydrocarbon molecules selected from an established set (Cioslowski et al., J. Chem. Phys. 2000, 113, 9377). This set of thermodynamic values includes ΔHfs for hydrocarbons that span a range of molecular sizes, degrees of aromaticity, and geometrical configurations, and, as such, provides a rigorous assessment of ccCAs applicability to a variety of hydrocarbons. The ΔHfs were calculated via atomization energies, isodesmic reactions, and hypohomodesmotic reactions. In addition, for 12 of the aromatic molecules in the set that are larger than benzene, the energies of ring‐conserved isodesmic reactions were used to calculate the ΔHf′s. Using an atomization energy approach to determine the ΔHf′s, the lowest mean absolute deviation (MAD) from experiment achieved by ccCA for the 60 hydrocarbons was 1.10 kcal mol−1. The use of the mixed Gaussian/inverse exponential complete basis set extrapolation scheme (ccCA‐P) in conjunction with hypohomodesmotic reaction energies resulted in a MAD of 0.87 kcal mol−1. This value is compared with MADs of 1.17, 1.18, and 1.28 kcal mol−1 obtained via the Gaussian‐4 (G4), Gaussian‐3 (G3), and Gaussian‐3(MP2) [G3(MP2)] methods, respectively (using the hypohomodesmotic reactions).

A non-classical copper carbonyl on a tri-alkene hydrocarbon support

Two rare copper complexes, a tri-alkene adduct [Cu(ttt-cdt)(FSbF(5))] and a non-classical carbon monoxide complex [Cu(ttt-cdt)(CO)][SbF(6)] with a high CO stretching frequency ([small nu, Greek, macron](CO) = 2160 cm(-1)), have been isolated using trans,trans,trans-1,5,9-cyclododecatriene (ttt-cdt) ligand.

Complete basis set limits of local second-order Møller–Plesset perturbation theory

The performance of local Møller–Plesset second-order perturbation theory (LMP2) and the impact of domain choice upon accuracy for a series of correlation consistent basis sets have been examined. MP2 correlation energies were calculated for 31 molecules ranging from 4 to 26 atoms, and containing up to 10 non-hydrogen atoms. The correlation energies were extrapolated to the complete basis set (CBS) limit using various schemes for comparison. The percent CPU savings for the local MP2 calculations as compared with conventional MP2 calculations are provided.