On the accuracy of spin-component-scaled perturbation theory (SCS-MP2) for the potential energy surface of the ethylene dimer

Spin-component-scaled and dispersion-corrected second-order Møller-Plesset perturbation theory: A path toward chemical accuracy

Second-order Moller-Plesset perturbation theory (MP2) provides a valuable alternative to density functional theory for modeing problems in organic and biological chemistry. However, MP2 suffers from known limitations in the description...

Prototypical π-π dimers re-examined by means of high-level CCSDT(Q) composite ab initio methods

The benzene-ethene and parallel-displaced (PD) benzene-benzene dimers are the most fundamental systems involving π-π stacking interactions. Several high-level ab initio investigations calculated the binding energies of these dimers using the coupled-cluster with singles, doubles, and quasi-perturbative triple excitations [CCSD(T)] method at the complete basis set [CBS] limit using various approaches such as reduced virtual orbital spaces and/or MP2-based basis set corrections. Here, we obtain CCSDT(Q) binding energies using a Weizmann-3-type approach. In particular, we extrapolate the self-consistent field (SCF), CCSD, and (T) components using large heavy-atom augmented Gaussian basis sets [namely, SCF/jul-cc-pV{5,6}Z, CCSD/jul-cc-pV{Q,5}Z, and (T)/jul-cc-pV{T,Q}Z]. We consider post-CCSD(T) contributions up to CCSDT(Q), inner-shell, scalar-relativistic, and Born-Oppenheimer corrections. Overall, our best relativistic, all-electron CCSDT(Q) binding energies are ∆E_e,all,rel = 1.234 (benzene-ethene) and 2.550 (benzene-benzene PD), ∆H₀ = 0.949 (benzene-ethene) and 2.310 (benzene-benzene PD), and ∆H₂₉₈ = 0.130 (benzene-ethene) and 1.461 (benzene-benzene PD) kcal mol^-1. Important conclusions are reached regarding the basis set convergence of the SCF, CCSD, (T), and post-CCSD(T) components. Explicitly correlated calculations are used as a sanity check on the conventional binding energies. Overall, post-CCSD(T) contributions are destabilizing by 0.028 (benzene-ethene) and 0.058 (benzene-benzene) kcal mol^-1, and thus, they cannot be neglected if sub-chemical accuracy is sought (i.e., errors below 0.1 kcal mol^-1). CCSD(T)/aug-cc-pwCVTZ core-valence corrections increase the binding energies by 0.018 (benzene-ethene) and 0.027 (benzene-benzene PD) kcal mol^-1. Scalar-relativistic and diagonal Born-Oppenheimer corrections are negligibly small. We use our best CCSDT(Q) binding energies to evaluate the performance of MP2-based, CCSD-based, and lower-cost composite ab initio procedures for obtaining these challenging π-π stacking binding energies.

Performance of DFT for C60 Isomerization Energies: A Noticeable Exception to Jacob's Ladder

The ability to accurately calculate relative energies of fullerenes is important in many areas of computational nanotechnology. Because of the large size of fullerenes, their relative energies cannot normally be calculated by means of high-level ab initio procedures, and therefore, density functional theory (DFT) represents a cost-effective alternative. In an extensive benchmark study, we calculate the electronic energies of eight C₆₀ isomers by means of the high-level G4(MP2) composite procedure. G4(MP2) isomerization energies span a wide range between 307.5 and 1074.0 kJ mol^-1. We use these benchmark data to assess the performance of DFT, double-hybrid DFT (DHDFT), and MP2-based ab initio methods. Surprisingly, functionals from the second and third rungs of Jacob's Ladder (i.e., GGA and meta-GGA functionals) significantly and systematically outperform hybrid and hybrid-meta-GGA functionals, which occupy higher rungs of Jacob's Ladder. In addition, DHDFT functionals do not offer a substantial improvement over meta-GGA functionals, with respect to isomerization energies. Overall, the best performing functionals with mean absolute deviations (MADs) below 15.0 kJ mol^-1 are (MADs given in parentheses) the GGA N12 (14.7); meta-GGAs M06-L (10.6), M11-L (10.8), MN15-L (11.9), and TPSS-D3BJ (12.8); and the DHDFT functionals B2T-PLYP (9.3), mPW2-PLYP (9.8), B2K-PLYP (12.1), and B2GP-PLYP (12.3 kJ mol^-1). In light of these results, we recommend the use of meta-GGA functionals for the calculation of fullerene isomerization energies. Finally, we show that inclusion of very small percentages of exact Hartree-Fock exchange (3-5%) slightly improves the performance of the GGA and meta-GGA functionals. However, their performance rapidly deteriorates with the inclusion of larger percentages of exact Hartree-Fock exchange.

Dynamically adjustable spin component scaled second order Møller-Plesset perturbation theory for strongly correlated molecular systems

We present a novel spin-component scaled Møller-Plesset second order (MP2) perturbation theory [S. Grimme, J. Chem. Phys. 118, 9095 (2003)] in which the singlet and triplet correlation channels are dressed in a dynamical manner over the entire molecular potential energy surface. In order to strike the right balance between the short and long range correlation, the different correlation channels are scaled by two complementary functions without introducing any external parameter: while the singlet channel contribution to correlation energy is attenuated with increasing strong correlation of the system, the triplet channel contribution is amplified. We have justified our approach from physical reasoning as well as a few numerical examples with some difficult systems, like symmetric stretching of water and nitrogen molecules, which clearly demonstrate the efficacy of this method in describing the molecular potential energy surface, even in the strongly correlated regions where the conventional MP2 and its other variants disastrously fail.

Stabilizing and Modulating Color by Copigmentation: Insights from Theory and Experiment

Natural anthocyanin pigments/dyes and phenolic copigments/co-dyes form noncovalent complexes, which stabilize and modulate (in particular blue, violet, and red) colors in flowers, berries, and food products derived from them (including wines, jams, purees, and syrups). This noncovalent association and their electronic and optical implications constitute the copigmentation phenomenon. Over the past decade, experimental and theoretical studies have enabled a molecular understanding of copigmentation. This review revisits this phenomenon to provide a comprehensive description of the nature of binding (the dispersion and electrostatic components of π-π stacking, the hydrophobic effect, and possible hydrogen-bonding between pigment and copigment) and of spectral modifications occurring in copigmentation complexes, in which charge transfer plays an important role. Particular attention is paid to applications of copigmentation in food chemistry.

Interactions of Molecules with cis and trans Double Bonds: A Theoretical Study of cis- and trans-2-Butene

Noncovalent interactions of cis- and trans-2-butene, as the smallest model systems of molecules with cis and trans double bonds, were studied to find potential differences in interactions of these molecules. The study was performed using quantum chemical methods including very accurate CCSD(T)/CBS method. We studied parallel and displaced parallel interactions in 2-butene dimers, in butane dimers, and between 2-butene and saturated butane. The results show the trend that interactions of 2-butene with butane are the strongest, followed by interactions in butane dimers, whereas the interaction in 2-butene dimers are the weakest. The strongest calculated interaction energy is between trans-2-butene and butane, with a CCSD(T)/CBS energy of -2.80 kcal mol(-1) . Interactions in cis-2-butene dimers are stronger than interactions in trans-2-butene dimers. Interestingly, some of the interactions involving 2-butene are as strong as interactions in a benzene dimer. These insights into interactions of cis- and trans-2-butene can improve understanding of the properties and processes that involve molecules with cis and trans double bonds, such as fatty acids and polymers.

Spin component scaling in multiconfiguration perturbation theory

We investigate a term-by-term scaling of the second-order energy correction obtained by perturbation theory (PT) starting from a multiconfiguration wave function. The total second-order correction is decomposed into several terms, based on the level and the spin pattern of the excitations. To define individual terms, we extend the same spin/different spin categorization of spin component scaling in various ways. When needed, identification of the excitation level is facilitated by the pivot determinant underlying the multiconfiguration PT framework. Scaling factors are determined from the stationary condition of the total energy calculated up to order 3. The decomposition schemes are tested numerically on the example of bond dissociation profiles and energy differences. We conclude that Grimme's parameters determined for single-reference Møller-Plesset theory may give a modest error reduction along the entire potential surface, if adopting a multireference based PT formulation. Scaling factors obtained from the stationary condition show relatively large variation with molecular geometry, at the same time they are more efficient in reducing the error when following a bond dissociation process.