The lowest singlet-triplet excitation energy of BN: a converged coupled cluster perspective

The notoriously small $X ^3\Pi-a ^1\Sigma^+$ excitation energy of the BN diatomic has been calculated using high-order coupled cluster methods. Convergence has been established in both the 1-particle basis set and the coupled cluster expansion. Explicit inclusion of connected quadruple excitations $\hat{T}_4$ is required for even semiquantitative agreement with the limit value, while connected quintuple excitations $\hat{T}_5$ still have an effect of about 60 cm$^{-1}$. Still higher excitations only account for about 10 cm$^{-1}$. Inclusion of inner-shell correlation further reduces $T_e$ by about 60 cm$^{-1}$ at the CCSDT, and 85 cm$^{-1}$ at the CCSDTQ level. Our best estimate, $T_e$=183$\pm$40 cm$^{-1}$, is in excellent agreement with earlier calculations and experiment, albeit with a smaller (and conservative) uncertainty. The dissociation energy of BN($X ^3\Pi$) is $D_e$=105.74$\pm$0.16 kcal/mol and $D_0$=103.57$\pm$0.16 kcal/mol.Comment: J. Chem. Phys., in pres

The S66 noncovalent interactions benchmark reconsidered using explicitly correlated methods near the basis set limit

The S66 benchmark for noncovalent interactions has been re-evaluated using explicitly correlated methods with basis sets near the one-particle basis set limit. It is found that post-MP2 "high-level corrections" are treated adequately well using a combination of CCSD(F12*) with (aug-)cc-pVTZ-F12 basis sets on the one hand, and (T) extrapolated from conventional CCSD(T)/heavy-aug-cc-pV{D,T}Z on the other hand. Implications for earlier benchmarks on the larger S66x8 problem set in particular, and for accurate calculations on noncovalent interactions in general, are discussed. At a slight cost in accuracy, (T) can be considerably accelerated by using sano-V{D,T}Z+ basis sets, while half-counterpoise CCSD(F12*)(T)/cc-pVDZ-F12 offers the best compromise between accuracy and computational cost.Comment: Australian Journal of Chemistry, in press [Graham S. Chandler special issue

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

The benzene...ethene and parallel-displaced (PD) benzene...benzene dimers are the most fundamental systems involving p-p stacking interactions. Several high-level ab initio investigations calculated the binding energies of these dimers at the CCSD(T)/CBS level of theory 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 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 Delta Ee,all,rel = 1.234 (benzene...ethene) and 2.550 (benzene...benzene PD), Delta H0 = 0.949 (benzene...ethene) and 2.310 (benzene...benzene PD), and Delta H298 = 0.130 (benzene...ethene) and 1.461 (benzene...benzene PD) kcal/mol. 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, thus they cannot be neglected if 0.1 kcal/mol accuracy is sought.Comment: J. Chem. Phys., accepted with minor revisio

Predicting the primary fragments in mass spectrometry using ab initio Roby–Gould bond indices

There is currently a lack of computational methods supporting the elucidation of unknown compounds by mass spectrometry. In this study, we develop and evaluate seven different protocols, based on the ab initio Roby–Gould bond indices [Gould et al., Theor. Chem. Acc., 2008, 119, 275] for predicting the mass-to-charge ratio of the highest intensity peak (base peak) in electron impact mass spectra. The protocols are applied to a dataset of 75 molecules, including five directly targeted semiochemicals. The Roby–Gould bond indices are also surveyed exhaustively, for the first time, for a dataset of 103 molecules with 682 CAC bonds. For neutral species we find that the bond indices are, as may be expected, highly correlated with the bond length; for cations, although there is a correlation, the bond indices are more variable. One of our protocols, protocol MG, correctly predicts the base peak in the mass spectra for 65 out of 75 cases. The correct base peak was calculated for three out of five targeted natural products.KA is grateful to the higher committee for education development in Iraq (HCED) for the award of a PhD scholarship. BB acknowledges funding from the Australian Research Council for grant DE160101313

Pinning the most stable H x C y O z isomers in space by means of high-level theoretical procedures

a b s t r a c t It has been recently demonstrated that there is a high statistical correlation between the relative energies of isomers and their relative abundances in the interstellar medium (ISM). In the present work we use the high-level W2-F12 thermochemical protocol to obtain accurate isomerization energies for a set of 109 H x C y O z isomers, 18 of which have been observed in the ISM so far. We use our benchmark isomerization energies to (i) rationalize the presence of the isomers that have been detected, and (ii) predict which new isomers are likely to be detected in the future. We find that the energetically most stable isomers of H 2 C 3 O (1,2-propadien-1-one), H 8 C 3 O (2-propanol), and H 6 C 3 O 2 (propanoic acid) have not been observed, despite the fact that higher-energy isomers of these chemical formulas have been detected in the ISM. The dipole moments of these isomers are sufficiently large that these species should be observed using microwave spectroscopy techniques

Estimating the CCSD basis-set limit energy from small basis sets: basis-set extrapolations vs additivity schemes

Coupled cluster calculations with all single and double excitations (CCSD) converge exceedingly slowly with the size of the one-particle basis set. We assess the performance of a number of approaches for obtaining CCSD correlation energies close to the complete basis-set limit in conjunction with relatively small DZ and TZ basis sets. These include global and system-dependent extrapolations based on the A + B/Lα two-point extrapolation formula, and the well-known additivity approach that uses an MP2-based basis-set-correction term. We show that the basis set convergence rate can change dramatically between different systems(e.g.it is slower for molecules with polar bonds and/or second-row elements). The system-dependent basis-set extrapolation scheme, in which unique basis-set extrapolation exponents for each system are obtained from lower-cost MP2 calculations, significantly accelerates the basis-set convergence relative to the global extrapolations. Nevertheless, we find that the simple MP2-based basis-set additivity scheme outperforms the extrapolation approaches. For example, the following root-mean-squared deviations are obtained for the 140 basis-set limit CCSD atomization energies in the W4-11 database: 9.1 (global extrapolation), 3.7 (system-dependent extrapolation), and 2.4 (additivity scheme) kJ mol–1. The CCSD energy in these approximations is obtained from basis sets of up to TZ quality and the latter two approaches require additional MP2 calculations with basis sets of up to QZ quality. We also assess the performance of the basis-set extrapolations and additivity schemes for a set of 20 basis-set limit CCSD atomization energies of larger molecules including amino acids, DNA/RNA bases, aromatic compounds, and platonic hydrocarbon cages. We obtain the following RMSDs for the above methods: 10.2 (global extrapolation), 5.7 (system-dependent extrapolation), and 2.9 (additivity scheme) kJ mol–1

From Molecules with a Planar Tetracoordinate Carbon to an Astronomically Known C5H2 Carbene

Ethynylcyclopropenylidene (2), an isomer of C5H2, is a known molecule in the laboratory and has recently been identified in Taurus Molecular Cloud-1 (TMC-1). Using high-level coupled-cluster methods up to the CCSDT(Q)/ CBS level of theory, it is shown that two isomers of C5H2 with a planar tetracoordinate carbon (ptC) atom, (SP-4)-spiro[2.2]pent-1,4-dien-1,4-diyl (11) and (SP-4)-spiro[2.2]pent-1,4-dien-1,5-diyl (13), serve as the reactive intermediates for the formation of 2. Here, a theoretical connection has been established between molecules containing ptC atoms (11 and 13) and a molecule (2) that is present nearly 430 light years away, thus providing evidence for the existence of ptC species in the interstellar medium. The reaction pathways connecting the transition states and the reactants and products have been confirmed by intrinsic reaction coordinate calculations at the CCSDT(Q)/CBS//B3LYP-D3BJ/cc-pVTZ level. While isomer 11 is non-polar (μ = 0), isomers 2 and 13 are polar, with dipole moment values of 3.52 and 5.17 Debye at the CCSD(T)/cc-pVTZ level. Therefore, 13 is also a suitable candidate for both laboratory and radioastronomical studies