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In two recent papers [A. D. Becke, J. Chem. Phys. 156, 214101 (2022) and A. D. Becke, J. Chem. Phys. 157, 234102 (2022)], we compared two Kohn–Sham density functionals based on physical modeling and theory with the best density-functional power-series fits in the literature. The best error statistics reported to date for a hybrid functional on the general main-group thermochemistry, kinetics, and noncovalent interactions (GMTKN55) chemical database of Goerigk et al. [Phys. Chem. Chem. Phys. 19, 32184 (2017)] were obtained. In the present work, additional second-order perturbation-theory terms are considered. The result is a 12-parameter double-hybrid density functional with the lowest GMTKN55 WTMAD2 "weighted total mean absolute deviation" error (1.76 kcal/mol) yet seen for any hybrid or double-hybrid density-functional approximation. We call it "DH23." [ABSTRACT FROM AUTHOR]

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 ∆Ee,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. [ABSTRACT FROM AUTHOR]

While the title question is a clear "yes" from purely theoretical arguments, the case is less clear for practical calculations with finite (one-particle) basis sets. To shed further light on this issue, the convergence to the basis set limit of CCSD (coupled cluster theory with all single and double excitations) and of different approximate implementations of CCSD-F12 (explicitly correlated CCSD) has been investigated in detail for the W4-17 thermochemical benchmark. Near the CBS ([1-particle] complete basis set) limit, CCSD and CCSD(F12*) agree to within their respective uncertainties (about ±0.04 kcal/mol) due to residual basis set incompleteness error, but a nontrivial difference remains between CCSD-F12b and CCSD(F12*), which is roughly proportional to the degree of static correlation. The observed basis set convergence behavior results from the superposition of a rapidly converging, attractive, CCSD[F12]–CCSD-F12b difference (consisting mostly of third-order terms) and a more slowly converging, repulsive, fourth-order difference between CCSD(F12*) and CCSD[F12]. For accurate thermochemistry, we recommend CCSD(F12*) over CCSD-F12b if at all possible. There are some indications that the nZaPa family of basis sets exhibits somewhat smoother convergence than the correlation consistent family. [ABSTRACT FROM AUTHOR]

We have developed a new basis set family, denoted as aug-cc-pVnZ-F12 (or aVnZ-F12 for short), for explicitly correlated calculations. The sets included in this family were constructed by supplementing the corresponding cc-pVnZ-F12 sets with additional diffuse functions on the higher angular momenta (i.e., additional d-h functions on non-hydrogen atoms and p-g on hydrogen atoms), optimized for the MP2-F12 energy of the relevant atomic anions. The new basis sets have been benchmarked against electron affinities of the first- and second-row atoms, the W4-17 dataset of total atomization energies, the S66 dataset of noncovalent interactions, the Benchmark Energy and Geometry Data Base water cluster subset, and the WATER23 subset of the GMTKN24 and GMTKN30 benchmark suites. The aVnZ-F12 basis sets displayed excellent performance, not just for electron affinities but also for noncovalent interaction energies of neutral and anionic species. Appropriate CABSs (complementary auxiliary basis sets) were explored for the S66 noncovalent interaction benchmark: between similarsized basis sets, CABSs were found to be more transferable than generally assumed. [ABSTRACT FROM AUTHOR]

In the context of high-accuracy computational thermochemistry, the valence coupled cluster with all singles and doubles (CCSD) correlation component of molecular atomization energies presents the most severe basis set convergence problem, followed by the (T) component. In the present paper, we make a detailed comparison, for an expanded version of the W4-11 thermochemistry benchmark, between, on the one hand, orbital-based CCSD/AV{5,6}Z+d and CCSD/ACV{5,6}Z extrapolation, and on the other hand CCSD-F12b calculations with cc-pVQZ-F12 and cc-pV5Z-F12 basis sets. This latter basis set, now available for H-He, B-Ne, and Al-Ar, is shown to be very close to the basis set limit. Apparent differences (which can reach 0.35 kcal/mol for systems like CCl4) between orbital-based and CCSD-F12b basis set limits disappear if basis sets with additional radial flexibility, such as ACV{5,6}Z, are used for the orbital calculation. Counterpoise calculations reveal that, while total atomization energies with V5Z-F12 basis sets are nearly free of BSSE, orbital calculations have significant BSSE even with AV(6+d)Z basis sets, leading to non-negligible differences between raw and counterpoise-corrected extrapolated limits. This latter problem is greatly reduced by switching to ACV{5,6}Z core-valence basis sets, or simply adding an additional zeta to just the valence orbitals. Previous reports that all-electron approaches like HEAT (high-accuracy extrapolated ab-initio thermochemistry) lead to different CCSD(T) limits than "valence limit+CV correction" approaches like Feller-Peterson-Dixon and Weizmann-4 (W4) theory can be rationalized in terms of the greater radial flexibility of core-valence basis sets. For (T) corrections, conventional CCSD(T)/AV{Q,5}Z+d calculations are found to be superior to scaled or extrapolated CCSD(T)-F12b calculations of similar cost. For a W4-F12 protocol, we recommend obtaining the Hartree-Fock and valence CCSD components from CCSD-F12b/cc-pV{Q,5}Z-F12 calculations, but the (T) component from conventional CCSD(T)/aug'-cc-pV{Q,5}Z+d calculations using Schwenke's extrapolation; post-CCSD(T), core-valence, and relativistic corrections are to be obtained as in the original W4 theory. W4-F12 is found to agree slightly better than W4 with ATcT (active thermochemical tables) data, at a substantial saving in computation time and especially I/O overhead. A W4-F12 calculation on benzene is presented as a proof of concept. [ABSTRACT FROM AUTHOR]

In an attempt to extend the applicability of the W1 and W2 ab initio computational thermochemistry methods, we propose explicitly correlated versions thereof, denoted W1-F12 and W2-F12. In W2-F12, we can 'save' one cardinal number (viz., angular momentum) in the basis set sequences without loss in accuracy; in W1-F12, we can do so for first-row compounds but not for second-row compounds. At a root mean square deviation (RMSD) = 0.19 kcal/mol for the first-row molecules in the W4-11 benchmark dataset, W1-F12 is in fact superior to ordinary W1 theory. For the entire W4-11 set, W2-F12 yields a RMSD = 0.20 kcal/mol, comparable to 0.19 kcal/mol from ordinary W2 theory. The extended applicability ranges of W1-F12 and W2-F12 are not just due to the lower computational cost but also to greatly reduced memory and especially storage requirements. They are illustrated through applications to nucleic acids and to polyacenes (with up to four rings), for which the following revised gas-phase heats of formation are found: ΔfH298° = 19.6 (benzene), 34.94 (naphthalene), 53.9, (anthracene), 73.9 (naphthacene/tetracene), 54.9 (adenine), -16.3 (cytosine), 5.1 (guanine), -80.6 (thymine), and -71.6 (uracil) kcal/mol. Our theoretical values for the DNA/RNA bases largely confirm recent predictions based on much lower-level calculations. The W1-F12 theoretical values for benzene, naphthalene, and anthracene are in very good to reasonable agreement with experiment. However, both W1-F12 and other computational estimates show that the accepted experimental value for naphthacene cannot be reconciled with those for the lower acenes: we suggest that ΔfH298°[naphthacene(g)] = 74.25 ± 1 kcal/mol is a more realistic estimate. [ABSTRACT FROM AUTHOR]

Accurate spectroscopic constants and electrical properties of small molecules are determined by means of W4 and post-W4 theories. For a set of 28 first- and second-row diatomic molecules for which very accurate experimental spectroscopic constants are available, W4 theory affords near-spectroscopic or better predictions. Specifically, the root-mean-square deviations (RMSDs) from experiment are 0.04 pm for the equilibrium bond distances (re), 1.03 cm-1 for the harmonic frequencies (ωe), 0.20 cm-1 for the first anharmonicity constants (ωexe), 0.10 cm-1 for the second anharmonicity constants (ωeye), and 0.001 cm-1 for the vibration-rotation coupling constants (αe). These RMSDs imply 95% confidence intervals of about 0.1 pm for re, 2.0 cm-1 for ωe, 0.4 cm-1 for ωexe, and 0.2 cm-1 for ωeye. We find that post-CCSD(T) contributions are essential to achieve such narrow confidence intervals for re and ωe, but have little effect on ωexe and αe, and virtually none on ωeye. Higher-order connected triples T⁁3-(T) improve the agreement with experiment for the hydride systems, but their inclusion (in the absence of T⁁4) tends to worsen the agreement with experiment for the nonhydride systems. Connected quadruple excitations T⁁4 have significant and systematic effects on re, ωe, and ωexe, in particular they universally increase re (by up to 0.5 pm), universally reduce ωe (by up to 32 cm-1), and universally increase ωexe (by up to 1 cm-1). Connected quintuple excitations T⁁5 are spectroscopically significant for ωe of the nonhydride systems, affecting ωe by up to 4 cm-1. Diagonal Born-Oppenheimer corrections have systematic and spectroscopically significant effects on re and ωe of the hydride systems, universally increasing re by 0.01-0.06 pm and decreasing ωe by 0.3-2.1 cm-1. Obtaining re and ωe of the pathologically multireference BN and BeO systems with near-spectroscopic accuracy requires large basis sets in the core-valence CCSD(T) step and augmented basis sets in the valence post-CCSD(T) steps in W4 theory. The triatomic molecules H2O, CO2, and O3 are also considered. The equilibrium geometries and harmonic frequencies (with the exception of the asymmetric stretch of O3) are obtained with near-spectroscopic accuracy at the W4 level. The asymmetric stretch of ozone represents a severe challenge to W4 theory, in particular the connected quadruple contribution converges very slowly with the basis set size. Finally, the importance of post-CCSD(T) correlation effects for electrical properties, namely, dipole moments (μ), polarizabilities (α), and first hyperpolarizabilities (β), is evaluated. [ABSTRACT FROM AUTHOR]

Basis set convergence of correlation effects on molecular atomization energies beyond the coupled cluster with singles and doubles (CCSD) approximation has been studied near the one-particle basis set limit. Quasiperturbative connected triple excitations, (T), converge more rapidly than L-3 (where L is the highest angular momentum represented in the basis set), while higher-order connected triples, T3-(T), converge more slowly—empirically, ∝L-5/2. Quasiperturbative connected quadruple excitations, (Q), converge smoothly as ∝L-3 starting with the cc-pVTZ basis set, while the cc-pVDZ basis set causes overshooting of the contribution in highly polar systems. Higher-order connected quadruples display only weak, but somewhat erratic, basis set dependence. Connected quintuple excitations converge very rapidly with the basis set, to the point where even an unpolarized double-zeta basis set yields useful numbers. In cases where fully iterative coupled cluster up to connected quintuples (CCSDTQ5) calculations are not an option, CCSDTQ(5) (i.e., coupled cluster up to connected quadruples plus a quasiperturbative connected quintuples correction) cannot be relied upon in the presence of significant nondynamical correlation, whereas CCSDTQ(5)Λ represents a viable alternative. Connected quadruples corrections to the core-valence contribution are thermochemically significant in some systems. We propose an additional variant of W4 theory [A. Karton et al., J. Chem. Phys. 125, 144108 (2006)], denoted W4.4 theory, which is shown to yield a rms deviation from experimental atomization energies (active thermochemical tables, ATcT) of only 0.05 kcal/mol for systems for which ATcT values are available. We conclude that “3σ≤=1 kJ/mol” thermochemistry is feasible with current technology, but that the more ambitious goal of ±10 cm-1 accuracy is illusory, at least for atomization energies. [ABSTRACT FROM AUTHOR]

In an attempt to improve on our earlier W3 theory [A. D. Boese et al., J. Chem. Phys. 120, 4129 (2004)] we consider such refinements as more accurate estimates for the contribution of connected quadruple excitations (T⁁4), inclusion of connected quintuple excitations (T⁁5), diagonal Born-Oppenheimer corrections (DBOC), and improved basis set extrapolation procedures. Revised experimental data for validation purposes were obtained from the latest version of the Active Thermochemical Tables thermochemical network. The recent CCSDT(Q) method offers a cost-effective way of estimating T⁁4, but is insufficient by itself if the molecule exhibits some nondynamical correlation. The latter considerably slows down basis set convergence for T⁁4, and anomalous basis set convergence in highly polar systems makes two-point extrapolation procedures unusable. However, we found that the CCSDTQ-CCSDT(Q) difference converges quite rapidly with the basis set, and that the formula 1.10[CCSDT(Q)/cc-pVTZ+CCSDTQ/cc-pVDZ-CCSDT(Q)/cc-pVDZ] offers a very reliable as well as fairly cost-effective estimate of the basis set limit T⁁4 contribution. The T⁁5 contribution converges very rapidly with the basis set, and even a simple double-zeta basis set appears to be adequate. The largest T⁁5 contribution found in the present work is on the order of 0.5 kcal/mol (for ozone). DBOCs are significant at the 0.1 kcal/mol level in hydride systems. Post-CCSD(T) contributions to the core-valence correlation energy are only significant at that level in systems with severe nondynamical correlation effects. Based on the accumulated experience, a new computational thermochemistry protocol for first- and second-row main-group systems, to be known as W4 theory, is proposed. Its computational cost is not insurmountably higher than that of the earlier W3 theory, while performance is markedly superior. Our W4 atomization energies for a number of key species are in excellent agreement (better than 0.1 kcal/mol on average, 95% confidence intervals narrower than 1 kJ/mol) with the latest experimental data obtained from Active Thermochemical Tables. Lower-cost variants are proposed: the sequence W1→W2.2→W3.2→W4lite→W4 is proposed as a converging hierarchy of computational thermochemistry methods. A simple a priori estimate for the importance of post-CCSD(T) correlation contributions (and hence a pessimistic estimate for the error in a W2-type calculation) is proposed. [ABSTRACT FROM AUTHOR]

A density functional theory exchange-correlation functional for the exploration of reaction mechanisms is proposed. This functional, denoted BMK (Boese-Martin for Kinetics), has an accuracy in the 2 kcal/mol range for transition state barriers but, unlike previous attempts at such a functional, this improved accuracy does not come at the expense of equilibrium properties. This makes it a general-purpose functional whose domain of applicability has been extended to transition states, rather than a specialized functional for kinetics. The improvement in BMK rests on the inclusion of the kinetic energy density together with a large value of the exact exchange mixing coefficient. For this functional, the kinetic energy density appears to correct “back” the excess exact exchange mixing for ground-state properties, possibly simulating variable exchange. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We are proposing a new computational thermochemistry protocol denoted W3 theory, as a successor to W1 and W2 theory proposed earlier [Martin and De Oliveira, J. Chem. Phys. 111, 1843 (1999)]. The new method is both more accurate overall (error statistics for total atomization energies approximately cut in half) and more robust (particularly towards systems exhibiting significant nondynamical correlation) than W2 theory. The cardinal improvement rests in an approximate account for post-CCSD(T) correlation effects. Iterative T[sub 3] (connected triple excitations) effects exhibit a basis set convergence behavior similar to the T[sub 3] contribution overall. They almost universally decrease molecular binding energies. Their inclusion in isolation yields less accurate results than CCSD(T) nearly across the board: It is only when T[sub 4] (connected quadruple excitations) effects are included that superior performance is achieved. T[sub 4] effects systematically increase molecular binding energies. Their basis set convergence is quite rapid, and even CCSDTQ/cc-pVDZ scaled by an empirical factor of 1.2532 will yield a quite passable quadruples contribution. The effect of still higher-order excitations was gauged for a subset of molecules (notably the eight-valence electron systems): T[sub 5] (connected quintuple excitations) contributions reach 0.3 kcal/mol for the pathologically multireference X [sup 1]Σ[sub g][sup +] state of C[sub 2] but are quite small for other systems. A variety of avenues for achieving accuracy beyond that of W3 theory were explored, to no significant avail. W3 thus appears to represent a good compromise between accuracy and computational cost for those seeking a robust method for computational thermochemistry in the kJ/mol accuracy range on small systems. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

When developing and assessing density functional theory methods, a finite basis set is usually employed. In most cases, however, the issue of basis set dependency is neglected. Here, we assess several basis sets and functionals. In addition, the dependency of the semiempirical fits to a given basis set for a generalized gradient approximation and a hybrid functional is investigated. The resulting functionals are then tested for other basis sets, evaluating their errors and transferability. [ABSTRACT FROM AUTHOR]

The total atomization energy at absolute zero, (TAE[sub 0]) of benzene, C[sub 6]H[sub 6], was computed fully ab initio by means of W2h theory as 1306.6 kcal/mol, to be compared with the experimentally derived value 1305.7±0.7 kcal/mol. The computed result includes contributions from inner-shell correlation (7.1 kcal/mol), scalar relativistic effects (-1.0 kcal/mol), atomic spin–orbit splitting (-0.5 kcal/mol), and the anharmonic zero-point vibrational energy (62.1 kcal/mol). The largest-scale calculations involved are CCSD/cc-pV5Z and CCSD(T)/cc-pVQZ; basis set extrapolations account for 6.3 kcal/mol of the final result. Performance of more approximate methods has been analyzed. Our results suggest that, even for systems the size of benzene, chemically accurate molecular atomization energies can be obtained from fully first-principles calculations, without resorting to corrections or parameters derived from experiment. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The quartic force field of cyclic C4 has been computed using basis sets of spdf quality and augmented coupled cluster methods. The effect of core correlation and further basis set extension has been investigated. Vibrational energy levels have been obtained using perturbation theory and two different variational approaches. A severe Fermi resonance exists between the most intense vibration, ν6, and ν3+ν5 through an exceptionally large k356=-258.2 cm-1; a large k1356 =-54.8 cm-1 causes significant higher-order anharmonicity, including a shift in ν6 of +9 cm-1. C4 appears to be an excellent test case for methods for solving the vibrational Schrödinger equation, since perturbation theory breaks down even when the above resonances are accounted for. Our best estimate for ν6, 1320±10 cm-1, may suggest its assignment to a feature detected at 1284 cm-1 in argon and 1302 cm-1 in krypton matrix, but this would imply an unusually large matrix red shift in argon. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The quartic force field of ethylene, C2H4, has been calculated ab initio using augmented coupled cluster, CCSD(T), methods and correlation consistent basis sets of spdf quality. For the 12C isotopomers C2H4, C2H3D, H2CCD2, cis-C2H2D2, trans-C2H2D2, C2HD3, and C2D4, all fundamentals are reproduced to better than 10 cm-1, except for three cases where the error is 11 cm-1. Our calculated harmonic frequencies suggest a thorough revision of the accepted experimentally derived values. Our computed and empirically corrected re geometry differs substantially from experimentally derived values: Both the predicted rz geometry and the ground-state rotational constants are, however, in excellent agreement with experiment, suggesting revision of the older values. Anharmonicity constants agree well with experiment for stretches, but differ substantially for stretch–bend interaction constants, due to equality constraints in the experimental analysis that do not hold. Improved criteria for detecting Fermi and Coriolis resonances are proposed and found to work well, contrary to the established method based on harmonic frequency differences that fails to detect several important resonances for C2H4 and its isotopomers. Surprisingly good results are obtained with a small spd basis at the CCSD(T) level. The well-documented strong basis set effect on the ν8 out-of-plane motion is present to a much lesser extent when correlation-optimized polarization functions are used. Complete sets of anharmonic, rovibrational coupling, and centrifugal distortion constants for the isotopomers are available as supplementary material to the paper via the World-Wide Web. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The total atomization energies of the carbon clusters Cn (n=2–10) have been calculated ab initio using augmented coupled-cluster methods and basis sets of spdf and spdfg quality. The values agree to within experimental uncertainty with recent Knudsen effusion measurements for Cn (n=2–7), but suggest actual values on the high side of their experimental range. For the odd-numbered clusters, an additivity approximation holds very well, and extrapolated ∑D0 values are proposed for C11, C13, C15, C17, and C19. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A very accurate ab initio quartic force field for CH4 and its isotopomers is presented. The quartic force field was determined with the singles and doubles coupled-cluster procedure that includes a quasiperturbative estimate of the effects of connected triple excitations, CCSD(T), using the correlation consistent polarized valence triple zeta, cc-pVTZ, basis set. Improved quadratic force constants were evaluated with the correlation consistent polarized valence quadruple zeta, cc-pVQZ, basis set. Fundamental vibrational frequencies are determined using second-order perturbation theory anharmonic analyses. All fundamentals of CH4 and isotopomers for which accurate experimental values exist and for which there is not a large Fermi resonance, are predicted to within ±6 cm-1. It is thus concluded that our predictions for the harmonic frequencies and the anharmonic constants are the most accurate estimates available. It is also shown that using cubic and quartic force constants determined with the correlation consistent polarized double zeta, cc-pVDZ, basis set in conjunction with the cc-pVQZ quadratic force constants and equilibrium geometry leads to accurate predictions for the fundamental vibrational frequencies of methane, suggesting that this approach may be a viable alternative for larger molecules. Using CCSD(T), core correlation is found to reduce the CH4 re by 0.0015 Å. Our best estimate for re is 1.0862±0.0005 Å. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The potential energy surface for the B2C molecule and the potential energy curve for the ground state of BC have been investigated using full-valence complete active space SCF (CASSCF), augmented coupled cluster [CCSD(T)] and multireference treatments. The ground state of B2C is an extraordinarily stable ring (∑ De=261.6±1 kcal/mol) with two 2-electron π systems. The first excited state is linear BCB (1Σ+g), which is essentially biconfigurational due to a (4σg)–(3σu) near degeneracy. Anharmonic spectroscopic constants were obtained from quartic force fields at the CCSD(T) level with a correlation-consistent basis set of [4s3p2d1f] quality. A severe Fermi resonance exists between the bending and symmetric stretching modes. All computed intensities are fairly weak. Spectroscopic constants for BC using elaborate multireference techniques were very well reproduced using the CCSD(T) method with a spin-restricted Hartree–Fock reference configuration, but not with an unrestricted Hartree–Fock reference. This suggests that even moderate levels of spin contamination that do not significantly affect relative energies may have a detrimental effect on computed spectroscopic constants. [ABSTRACT FROM AUTHOR]

The total atomization energies (∑De values), geometries, and harmonic frequencies for a number of experimentally well-described molecules have been calculated at the CCSD(T) (coupled cluster) level using Dunning’s correlation-consistent cc-pVDZ([3s2p1d]), cc-pVTZ([4s3p2d1f]), and cc-pVQZ([5s4p3d2f1g]) basis sets. Additivity correction are proposed for binding energies and geometries. Using a three-term additive correction of the form proposed by Martin [J. Chem. Phys. 97, 5012 (1992)] mean absolute errors in ∑De are 0.46 kcal/mol for the cc-pVQZ, 0.93 for the cc-pVTZ, and 2.59 for the c-pVDZ basis sets. The latter figure implies that, although unsuitable for quantitatively accurate work, three-term corrected CCSD(T)/cc-pVDZ binding energies can still be used for a rough estimate when the cost of larger basis set calculations would be prohibitive. CCSD(T)/cc-pVQZ calculations reproduce bond lengths to 0.001 Å for single bonds, and 0.003 Å for multiple bonds; remaining error is probably partly due to core–core and core–valence correlation. CCSD(T)/cc-pVTZ calculations result in additional overestimates of 0.001 Å for single, 0.003 Å for double, and 0.004 Å for triple bonds. CCSD(T)/cc-pVDZ calculations result in further overestimates of 0.01 Å for single bonds, and 0.02 Å for multiple bonds.CCSD(T)/cc-pVDZ harmonic frequencies are in surprisingly good agreement with experiment, except for pathological cases like the umbrella mode in NH3. Both CCSD(T)/cc-pVTZ and CCSD(T)/cc-pVQZ harmonic frequencies generally agree with experiment to 10 cm-1 or better; performance of cc-pVQZ is somewhat superior on multiple bonds or the umbrella mode in NH3. Again, a source of remaining error appears to be core correlation. The use of MP2/6-31G* reference geometries in the ∑De calculation can result in fairly substantial errors in the uncorrected ∑De values for systems with cumulated multiple bonds.... [ABSTRACT FROM AUTHOR]

Pulsed laser evaporation of pellets pressed from boron and graphite powder gave a new 1:4 doublet at 1232.5 and 1194.6 cm-1 in addition to the carbon cluster absorptions reported previously. The 1232.5 cm-1 band dominated boron-10 experiments. The new bands increased as carbon cluster bands decreased with increasing B/C ratio in the pellet and with increasing laser power. Augmented coupled cluster and full-valence complete active space SCF (CASSCF) calculations predict the global minimum BC2 structure to be an asymmetric triangle: however, the vibrationally averaged structure will be an isosceles triangle with a strong symmetric B–C2 stretching frequency near 1200 cm-1. The calculated boron-10/boron-11 frequency ratio (1.0323) is in excellent agreement with the observed ratio (1.0317), and confirms assignment of the 1194.6 cm-1 band to the BC2 ring. Calculations predict linear BCC to be less stable by 6.2±2 kcal/mol and to absorb in the 2000–2050 cm-1 range: the barrier towards rearrangement to the cyclic structure is very low (1.1 kcal/mol). Linear BCC was not detected in these experiments. Computed energetics explain why BC2 is abundant in B/C experiments, but absent in B+C2H2 experiments. [ABSTRACT FROM AUTHOR]

The notoriously small X 3Π-a 1Σ+ excitation energy of the BN diatomic has been calculated using high-order coupled cluster methods. Convergence has been established in both the one-particle basis set and the coupled cluster expansion. Explicit inclusion of connected quadruple excitations T⁁4 is required for even semiquantitative agreement with the limit value, while connected quintuple excitations 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 Te by about 60 cm-1 at the CCSDT, and 85 cm-1 at the CCSDTQ level. Our best estimate, Te=183±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 3Π) is De=105.74±0.16 kcal/mol and D0=103.57±0.16 kcal/mol. [ABSTRACT FROM AUTHOR]

The adiabatic electron affinity (AEA) of SF6 has been calculated near the relativistic CCSDT(Q) basis set limit. Our best theoretical value (1.0340 ± 0.03 eV) is in excellent agreement with the recently revised experimental value of 1.03 ± 0.05 eV reported by Troe et al. [J. Chem. Phys. 136, 121102 (2012)]. While our best nonrelativistic, clamped-nuclei, valence CCSD(T) basis set limit value of 0.9058 eV is in good accord with the previously reported CCSD(T)/CBS values, to obtain an accurate AEA, several additional contributions need to be taken into account. The most important one is scalar-relativistic effects (0.0839 eV), followed by inner-shell correlation (0.0216 eV) and post-CCSD(T) correlation effects (0.0248 eV), the latter almost entirely due to connected quadruple excitations. The diagonal Born-Oppenheimer correction is an order of magnitude less important at -0.0022 eV. [ABSTRACT FROM AUTHOR]

The performance of two recent ab initio computational thermochemistry schemes, W1 and W2 theory [J. M. L. Martin and G. de Oliveira, J. Chem. Phys. 111, 1843 (1999)], is assessed for an enlarged sample of thermochemical data consisting of the ionization potentials and electron affinities in the G2-1 and G2-2 sets, as well as the heats of formation in the G2-1 and a subset of the G2-2 set. We find W1 theory to be several times more accurate for ionization potentials and electron affinities than commonly used (and less expensive) computational thermochemistry schemes such as G2, G3, and CBS-QB3: W2 theory represents a slight improvement for electron affinities but no significant one for ionization potentials. The use of a two-point A+B/L[sup 5] rather than a three-point A+B/C[sup L] extrapolation for the self-consistent field (SCF) component greatly enhances the numerical stability of the W1 method for systems with slow basis set convergence. Inclusion of first-order spin-orbit coupling is essential for accurate ionization potentials and electron affinities involving degenerate electronic states: Inner-shell correlation is somewhat more important for ionization potentials than for electron affinities, while scalar relativistic effects are required for the highest accuracy. The mean deviation from experiment for the G2-1 heats of formation is within the average experimental uncertainty. W1 theory appears to be a valuable tool for obtaining benchmark quality proton affinities. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We propose large-core correlation-consistent (cc) pseudopotential basis sets for the heavy p-block elements Ga-Kr and In-Xe. The basis sets are of cc-pVTZ and cc-pVQZ quality, and have been optimized for use with the large-core (valence-electrons only) Stuttgart-Dresden-Bonn (SDB) relativistic pseudopotentials. Validation calculations on a variety of third-row and fourth-row diatomics suggest them to be comparable in quality to the all-electron cc-pVTZ and cc-pVQZ basis sets for lighter elements. Especially the SDB-cc-pVQZ basis set in conjunction with a core polarization potential (CPP) yields excellent agreement with experiment for compounds of the later heavy p-block elements. For accurate calculations on Ga (and, to a lesser extent, Ge) compounds, explicit treatment of 13 valence electrons appears to be desirable, while it seems inevitable for In compounds. For Ga and Ge, we propose correlation consistent basis sets extended for (3d) correlation. For accurate calculations on organometallic complexes of interest to homogenous catalysis, we recommend a combination of the standard cc-pVTZ basis set for first- and second-row elements, the presently derived SDB-cc-pVTZ basis set for heavier p-block elements, and for transition metals, the small-core [6s5p3d] Stuttgart-Dresden basis set-relativistic effective core potential combination supplemented by (2f1g) functions with exponents given in the Appendix to the present paper. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Core correlation and scalar relativistic contributions to the atomization energy of 120 first- and second-row molecules have been determined using coupled cluster and averaged coupled-pair functional methods and the MTsmall core correlation basis set. These results are used to parametrize an improved version of a previously proposed bond order scheme for estimating contributions to atomization energies. The resulting model, which requires negligible computational effort, reproduces the computed core correlation contributions with 88%-94% average accuracy (depending on the type of molecule), and the scalar relativistic contribution with 82%-89% accuracy. This permits high-accuracy thermochemical calculations at greatly reduced computational cost. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Accurate quartic anharmonic force fields for CF[sub 4] and SiF[sub 4] have been calculated using the CCSD(T) method and basis sets of spdf quality. Based on the ab initio force field with a minor empirical adjustment, the vibrational energy levels of these two molecules and their isotopomers are calculated by means of high order Canonical Van Vleck Perturbation Theory (CVPT) based on curvilinear coordinates. The calculated energies agree very well with the experimental data. The full quadratic force field of CF[sub 4] is further refined to the experimental data. The symmetrization of the Cartesian basis for arbitrary combination bands of T[sub d] group molecules is discussed using the circular promotion operator for the doubly degenerate modes, together with tabulated vector coupling coefficients. The extraction of the spectroscopic constants from our second order transformed Hamiltonian in curvilinear coordinates is discussed, and compared to a similar procedure in rectilinear coordinates. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The basis set convergence of explicitly correlated double-hybrid density functional theory (DFT) is investigated using the B2GP-PLYP functional. As reference values, we use basis set limit B2GP-PLYP-F12 reaction energies extrapolated from the aug(')-cc-pV(Q+d)Z and aug(')-cc-pV(5+d)Z basis sets. Explicitly correlated double-hybrid DFT calculations converge significantly faster to the basis set limit than conventional calculations done with basis sets saturated up to the same angular momentum (typically, one "gains" one angular momentum in the explicitly correlated calculations). In explicitly correlated F12 calculations the VnZ-F12 basis sets converge faster than the orbital A(')VnZ basis sets. Furthermore, basis set convergence of the MP2-F12 component is apparently faster than that of the underlying Kohn-Sham calculation. Therefore, the most cost-effective approach consists of combining the MP2-F12 correlation energy from a comparatively small basis set such as VDZ-F12 with a DFT energy from a larger basis set such as aug(')-cc-pV(T+d)Z., (© 2011 American Institute of Physics)