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The f-block ab initio correlation consistent composite approach was used to predict the dissociation energies of lanthanide sulfides and selenides. Geometry optimizations were carried out using density functional theory and coupled cluster singles, doubles, and perturbative triples with one- and two-component Hamiltonians. For the two-component calculations, relativistic effects were accounted for by utilizing a third-order Douglas–Kroll–Hess Hamiltonian. Spin–orbit coupling was addressed with the Breit–Pauli Hamiltonian within a multireference configuration interaction approach. The state averaged complete active space self-consistent field wavefunctions obtained for the spin–orbit coupling energies were used to assign the ground states of diatomics, and several diagnostics were used to ascertain the multireference character of the molecules.

The multi-configuration electron–nuclear dynamics for open shell systems with a spin-unrestricted formalism is described. The mean fields are evaluated using second-order reduced density matrices for electronic and nuclear degrees of freedom. Applications to light-element diatomics including equilibrium geometries, electronic energies, dipole moments, and absorption spectra are presented. The von Neumann entropies for different spin states of a LiH molecule are compared.

High level multireference calculations were performed for LuF for a total of 132 states, including four dissociation channels Lu(2D) + F(2P), Lu(2P) + F(2P), and two Lu(4F) + F(2P). The 6s, 5d, and 6p orbitals of lutetium, along with the valence 2p and 3p orbitals of fluorine, were included in the active space, allowing for the accurate description of static and dynamic correlation. The Lu(4F) + F(2P) channel has intersystem spin crossings with the Lu(2P) + F(2P) and Lu(2D) + F(2P) channels, which are discussed herein. To obtain spectroscopic constants, bond lengths, and excited states, multi-reference configuration interaction (MRCI) was used at a quadruple-ζ basis set level, correlating also the 4f electrons and corresponding orbitals. Core spin–orbit (C-MRCI) calculations were performed, revealing that 13Π0− is the first excited state closely followed by 13Π0+. In addition, the dissociation energy of LuF was determined at different levels of theory, with a range of basis sets. A balance between core correlation and a relativistic treatment of electrons is fundamental to obtain an accurate description of the dissociation energy. The best prediction was obtained with a combination of coupled-cluster single, double, and perturbative triple excitations /Douglas–Kroll–Hess third order Hamiltonian methods at a complete basis set level with a zero-point energy correction, which yields a dissociation value of 170.4 kcal mol−1. Dissociation energies using density functional theory were calculated using a range of functionals and basis sets; M06-L and B3LYP provided the closest predictions to the best ab initio calculations.

With the recent advances in experimental attosecond science, theoretical predictions of electron dynamics can now be validated against experiment. Time-dependent studies of the electron motion in molecules can be used to obtain information about electronic transitions and the interaction of the electrons with electromagnetic fields. Often, these approaches rely on single-excited wave functions. Presented here is a first attempt to evaluate the accuracy of the time-dependent configuration interaction method so that the optimal representation of the electronic wave function for time-dependent studies can be assessed. A quantifier is determined that can aid in finding this optimal representation. The approach is demonstrated on a variety of molecules that include both localized and intramolecular charge transfer electron excitations. Observables including excitation energies, dipole moments, strengths, and static polarizabilities are obtained from time-independent and time-dependent calculations and are compared to experimental data. In this way, a rigorous routine is developed by which the reliability and accuracy of the CI wave function can be assessed and which represents a first step to a more quantitative description of electron dynamics in molecules.

The cc-pV(n+d)Z correlation consistent basis sets of double- through quintuple-ζ quality for the atoms Al-Ar have been modified for use with density functional theory (DFT). These basis set modifications include truncation of high-angular momentum basis functions, recontraction of the s- and p-functions, and reoptimization of basis function exponents with generalized gradient approximation and hybrid-DFT functionals. The effects of basis set truncation, recontraction, and reoptimization are shown to improve convergence behavior in atomic energies as well as dissociation energies and enthalpies of formation.

In this work, the correlation consistent family of Gaussian basis sets has been expanded to include all-electron basis sets for In–Xe. The methodology for developing these basis sets is described, and several examples of the performance and utility of the new sets have been provided. Dissociation energies and bond lengths for both homonuclear and heteronuclear diatomics demonstrate the systematic convergence behavior with respect to increasing basis set quality expected by the family of correlation consistent basis sets in describing molecular properties. Comparison with recently developed correlation consistent sets designed for use with the Douglas-Kroll Hamiltonian is provided.

The ground and low-lying excited state potential energy curves of neodymium monofluoride were calculated using multireference (CASSCF) and single reference (EOM-CR-CCSD(T)) methods. Optimized bond lengths were obtained and accurate bond dissociation energies were computed. The EOM-CR-CCSD(T) method was used to determine the bond dissociation energy of lutetium monofluoride, and it is shown that core correlation is required to produce bond dissociation energies in agreement with experiment.

For molecules containing second row atoms, unacceptable errors have been found in extrapolating dissociation energies calculated with the standard correlation consistent basis sets to the complete basis set limit. By carefully comparing the convergence behavior of De(O2) and De(SO), we show that the cause of these errors is a result of two inter-related problems: near duplication of the exponents in two of the d sets and a lack of high-exponent functions in the early members of the sets. Similar problems exist for the f sets (and probably in higher angular momentum sets), but have only a minor effect on the calculated dissociation energies. A number of approaches to address the problems in the d sets were investigated. Well behaved convergence was obtained by augmenting the (1d) and (2d) sets with a high-exponent function and by replacing the (3d) set by the (4d) set and the (4d) set by the (5d) set and so on. To ensure satisfactory coverage of both the L and M shell regions, the exponents of the new d se...

Valence correlation consistent and augmented correlation consistent basis sets have been determined for the third row, main group atoms gallium through krypton. The methodology, originally developed for the first row atoms, was first applied to the selenium atom, resulting in the expected natural groupings of correlation functions (although higher angular momentum functions tend to be relatively more important for the third row atoms as they were for the second row atoms). After testing the generality of the conclusions for the gallium atom, the procedure was used to generate correlation consistent basis sets for all of the atoms gallium through krypton. The correlation consistent basis sets for the third row main group atoms are as follows: cc-pVDZ: (14s11p6d)/[5s4p2d]; cc-pVTZ: (20s13p9d1f )/[6s5p3d1f]; cc-pVQZ: (21s16p12d2 f1g)/[7s6p4d2 f1g]; cc-pV5Z: (26s17p13d3f2g1h)/[8s7p5d3f2g1h]. Augmented sets were obtained by adding diffuse functions to the above sets (one for each angular momentum present in the set), with the exponents of the additional functions optimized in calculations on the atomic anions. Test calculations on the atoms as well as selected molecules with the new basis sets show good convergence to an apparent complete basis set limit.

The tunneling effects in the molecular inversion of aziridine are investigated. A full-dimensional potential was constructed based on ab initio results and used to study the effect of vibrational excitations on tunneling. Using the semiclassical approach that incorporates tunneling into classical trajectory calculations, it is found that excitations of all the modes except the reaction coordinate have negligible effects on tunneling. This allows accurate thermal rate calculations carried out by using a one-dimensional model. (This is in accord with the conclusions of the work by Rom et al. [Chem. Phys. Lett. 204, 175 (1993)] and by Smedarchina et al. [J. Chem. Phys. 102, 7024 (1995)].) A one-dimensional Weutzel–Kramers–Brillouin (WKB) formula was employed to obtain the level splitting and was found to be very accurate based on comparisons with the quantum results. Furthermore, the calculated thermal rates are in good agreement with the experimentally measured values. The results provide insight into the reaction mechanism and explanations for the experimental findings of non-Rice–Ramsperger–Kassel–Marcus (RRKM) behavior and incoherent tunneling.

The total energies, atomization energies, molecular structures, and harmonic vibrational frequencies of SO2 have been determined using the newly revised correlation consistent basis sets, cc-pV(n+d)Z, with n=2–5. The impact of the additional tight d functions on the energies, structures, and frequencies as well as the convergence of these quantities to the complete basis set (CBS) limit is examined. Overall, the revised sets result in faster convergence to the CBS limit. The effect of the additional tight d function is particularly dramatic for the lower level, double zeta and triple zeta, basis sets.

The relativistic pseudopotential variant of the correlation consistent composite approach (rp-ccCA) has been extended to lower p-block-containing molecules. The lower p-block is defined here as elements indium through iodine (In-I) and thallium through astatine (Tl-At). The accuracy of rp-ccCA has previously been established in a study of energetic properties of molecules containing 4p elements and thermodynamic data of 4d-containing molecules. In the current study, the atomic ionization potentials (IPs) and electron affinities (EAs) of In-I and Tl-At, as well as dissociation energies and enthalpies of formation of lower p-block mono-hydrides, mono-oxides, and dimers were determined with rp-ccCA. The compiled data set is referred to as the LP80 data set. The mean absolute deviation of the IPs and the EAs from the experimental data were 2.50 and 3.06 kcal mol(-1), respectively. Additionally, the total atomization energy for lead clusters (Pb(n), n = 2-6) was determined. For the lead clusters, a decrease in the total atomization energy was observed with the inclusion of spin-orbit coupling. The spin-orbit coupling of the molecules was determined with multireference configuration interaction, while scalar relativistic effects were accounted for within the pseudopotentials. The results obtained by rp-ccCA for the lower p-block support the utilization of the method in future studies where accurate energetic, spectroscopic, and thermodynamic data are necessary.

Theoretical studies, using the multireference correlation consistent composite approach (MR-ccCA), have been carried out on the ground and lowest lying spin-forbidden excited states of a series of silicon-containing systems. The MR-ccCA method is the multireference equivalent of the successful single reference ccCA method that has been shown to produce chemically accurate (within ±1.0 kcal mol(-1) of reliable, well-established experiment) results. The percentage contributions of the SCF configurations to complete active space self-consistent field wave functions together with the Frobenius norm of the t(1) vectors and related D(1) diagnostics of the coupled-cluster single double wave function with the cc-pVTZ basis set have been utilized to illustrate the multi-configurational characteristics of the compounds considered. MR-ccCA incorporates additive terms to account for relativistic effects, atomic spin-orbit coupling, scalar relativistic effects, and core-valence correlation. MR-ccCA has been utilized to predict the atomization energies, enthalpies of formation, and the lowest energy spin-forbidden transitions for Si(n)X(m) (2 ≤ n + m ≥ 3 where n ≠ 0 and X = B, C, N, Al, P), silicon hydrides, and analogous compounds of carbon. The energetics of small silicon aluminides and phosphorides are predicted for the first time.

A multireference analog of the correlation consistent composite approach (MR-ccCA) based on complete active space with second-order perturbation theory (CASPT2) has been utilized in an investigation of the ground and valence excited states of C(2), N(2), and O(2). The performance of different second-order multireference perturbation theory methods including second-order n-electron valence state perturbation theory, second-order multireference Møller-Plesset, and second-order generalized van Vleck perturbation theory has been analyzed as potential alternatives to CASPT2 within MR-ccCA. The MR-ccCA-P predicts spectroscopic constants with overall mean absolute deviations from experimental values of 0.0006 Å, 7.0 cm(-1), and 143 cm(-1) for equilibrium bond length (r(e)), harmonic frequency (ω(e)), and term values (T(e)), respectively, which are comparable to the predictions by more computationally costly multireference configuration interaction-based methods.

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.

Systematic truncation of the correlation consistent basis sets has been investigated in first and second row atoms and molecules to better understand basis set requirements for density functional theory, particularly the need for high angular momentum functions, as well as to understand possible computational cost savings that could be achieved by using reduced basis sets. The truncation scheme employed follows that recently introduced for ab initio methods [B. Mintz et al., J. Chem. Phys. 121, 5629 (2004)]. Properties examined in the current study include geometries, ionization potentials, electron affinities, and dissociation energies. In general, this investigation shows that a degree of truncation of higher angular momentum functions is possible with limited impact upon energetic properties, and does result in useful CPU time savings. However, not all properties investigated have the same level of dependence upon high angular momentum functions, and, thus, careful selection of truncated basis sets should be made.

An effective approach for the determination of lanthanide energetics, as demonstrated by application to the third ionization energy (in the gas phase) for the first half of the lanthanide series, has been developed. This approach uses a combination of highly correlated and fully relativistic ab initio methods to accurately describe the electronic structure of heavy elements. Both scalar and fully relativistic methods are used to achieve an approach that is both computationally feasible and accurate. The impact of basis set choice and the number of electrons included in the correlation space has also been examined.

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 coupled cluster approximation with single, double, and quasiperturbative triple excitations [CCSD(T)] was used in combination with the Douglas-Kroll contracted correlation consistent basis sets [cc-pVnZ-DK, where n = D(2), T(3), Q(4), and 5] and small-core relativistic pseudopotentials (PP) with correlation consistent polarized valence basis sets (cc-pVnZ-PP and aug-cc-pVnZ-PP) to investigate the impact of scalar relativistic corrections on energetic and structural properties of small molecules containing third-row (Ga-Kr) atoms. These molecules were taken from the Gaussian-2 extended test set for third-row atoms. Atomization energies, ionization energies, electron affinities, and proton affinities for molecules in the test set were determined and compared with nonrelativistic results which were obtained in a recent study in which the standard and augmented correlation consistent basis sets were used in combination with CCSD(T). Several schemes were used to extrapolate the energies to the complete basis set limit.

The systematic reduction of the commonly used correlation consistent basis sets [cc-pVnZ where n=D(2), T(3), Q(4), and 5] as a means to reduce computational cost has been extended to hydrogen-containing third-row (Ga–Kr) molecules of the G2 test suite. Coupled cluster with singles, doubles, and quasiperturbative triple excitations [CCSD(T)] calculations were performed using both the full correlation consistent basis sets and a series of truncated basis sets in order to assess the impact of basis set reduction upon the structures and energies of the species. The impact that truncation of the basis sets for hydrogen has upon extrapolation of energies to the complete basis set limit also has been examined, and the cost savings that can be achieved are discussed. Overall, basis set reduction can be accomplished which preserves the systematic convergence behavior of the full correlation consistent basis sets.

The systematic reduction of commonly used basis sets as a means to reduce computational cost is examined for a small test set of molecules, which includes H(2), CH(4), NH(3), H(2)O, HF, and HCN. Coupled cluster with single, double, and quasiperturbative triple excitations calculations were performed using both the correlation consistent basis sets, and a set of systematically reduced basis sets to examine both the impact of the reduction upon the accuracy of the structures and energies, and the computational cost savings achieved. The effect of several truncation scenarios upon basis set convergence is also examined. Overall, for the systems studied, a reduction can occur which preserves the well-established systematic convergence behavior of the correlation consistent basis sets.

Potential energy curves have been constructed using coupled cluster with singles, doubles, and perturbative triple excitations (CCSD(T)) in combination with all-electron and pseudopotential-based multiply augmented correlation consistent basis sets [m-aug-cc-pV(n + d)Z; m = singly, doubly, triply, n = D,T,Q,5]. The effect of basis set superposition error on the spectroscopic properties of Ar-Zn, Ar2-Zn, Ar-Ag(+), and Ar2-Ag(+) van der Waals complexes was examined. The diffuse functions of the doubly and triply augmented basis sets have been constructed using the even-tempered expansion. The a posteriori counterpoise scheme of Boys and Bernardi and its generalized variant by Valiron and Mayer has been utilized to correct for basis set superposition error (BSSE) in the calculated spectroscopic properties for diatomic and triatomic species. It is found that even at the extrapolated complete basis set limit for the energetic properties, the pseudopotential-based calculations still suffer from significant BSSE effects unlike the all-electron basis sets. This indicates that the quality of the approximations used in the design of pseudopotentials could have major impact on a seemingly valence-exclusive effect like BSSE. We confirm the experimentally determined equilibrium internuclear distance (re), binding energy (De), harmonic vibrational frequency (ωe), and C(1)Π ← X(1)Σ transition energy for ArZn and also predict the spectroscopic properties for the low-lying excited states of linear Ar2-Zn (X(1)Σg, (3)Πg, (1)Πg), Ar-Ag(+) (X(1)Σ, (3)Σ, (3)Π, (3)Δ, (1)Σ, (1)Π, (1)Δ), and Ar2-Ag(+) (X(1)Σg, (3)Σg, (3)Πg, (3)Δg, (1)Σg, (1)Πg, (1)Δg) complexes, using the CCSD(T) and MR-CISD + Q methods, to aid in their experimental characterizations.

The correlation consistent composite approach (ccCA) has proven to be an effective first-principles-based composite approach for main group and first-row transition metal species. By combining relativistic pseudopotentials and ccCA, accurate energetic and thermodynamic data for heavier elements, including transition metals, is obtainable. Relativistic pseudopotential ccCA (rp-ccCA) was formulated and tested on 25 molecules from the G3∕05 set that contain 4p elements (Ga-Kr). A 32.5% time savings was obtained using rp-ccCA, relative to ccCA employing all-electron basis sets. When implementing rp-ccCA to compute dissociation energies and enthalpies of formation for molecules from the 4p block, rp-ccCA results in a mean absolute deviation of 0.89 kcal mol(-1) from experimental data. rp-ccCA was also applied to a set of 30 4d transition metal-containing molecules, ranging from diatomics to Mo(CO)(6), and enthalpies of formation for these species were obtained with a mean absolute deviation of 2.89 kcal mol(-1) in comparison to experimental data. Based on quality of the experimentally available enthalpies of formation, where the average value of reported experimental error bars is 3.43 kcal mol(-1), rp-ccCA is within transition metal chemical accuracy for the 4d molecule set. rp-ccCA is a pseudopotential-based composite method for transition metals and is shown to yield accurate thermodynamic results for molecules containing heavy elements Ga-Kr and Y-Cd.

A new implementation of the correlation consistent composite approach (ccCA), denoted RI-ccCA, utilizing both the resolution of the identity (RI) and local methods is presented. A set of 102 molecules composed of first and second row, main group atoms is employed to compare total energies, atomization energies, and enthalpies of formation between the original ccCA implementation and those of RI-ccCA. Relative CPU time and disk space requirements of RI-ccCA as compared to ccCA, demonstrate that on the average, employing the RI approximation in ccCA affords CPU time savings over 70% and disk space requirements diminished by well over 90% without introducing significant error in energetic properties.

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.

Recent development of pseudopotential-based correlation consistent basis sets (cc-pVnZ-PP) by Peterson and Puzzarini [Theor. Chem. Acc. 114, 283 (2005)] has enabled the relative importance of metal versus ligand basis set size to be examined systematically. The impact of basis set choice on geometries and dissociation energies for a series of group 11 transition metal species has been assessed via three series of calculations: (1) systematically increasing the size of the cc-pVnZ-PP basis set on the metal while holding the basis set on the ligand constant, (2) systematically increasing the size of the cc-pVnZ basis set on the ligand while holding the basis set on the metal constant, and (3) systematically increasing the size of the basis set on both the metal and the ligand. Holding the ligand basis set static while systematically increasing the metal basis set results in changes in the equilibrium bond length that are an order of magnitude smaller than for calculations where the metal basis set is held constant and the quality of the ligand basis set is systematically increased. Interestingly, holding the metal basis set static while systematically increasing the basis set size on the ligand results in monotonic convergence of dissociation energies with respect to increasing basis set size, while maintaining the basis set size on the ligand and increasing the size of the basis set on the metal do not result in monotonic convergence. Also, variance of the ligand basis set size has a greater impact on the energetics than variance of the metal basis set size. This suggests that the choice of basis set for the ligands is much more important for accurate chemical description than the choice of the transition metal basis set for these species and properties. In fact, complete basis set limit dissociation energies obtained from increasing the size of the basis set on the ligand while maintaining a constant level basis set on the metal at any level basis set result in similar energies to those obtained utilizing large basis sets on both the metal and the ligand at significant computational cost savings.

Effects of increased basis-set size as well as a correlated treatment of the diagonal Born-Oppenheimer approximation are studied within the context of the high-accuracy extrapolated ab initio thermochemistry (HEAT) theoretical model chemistry. It is found that the addition of these ostensible improvements does little to increase the overall accuracy of HEAT for the determination of molecular atomization energies. Fortuitous cancellation of high-level effects is shown to give the overall HEAT strategy an accuracy that is, in fact, higher than most of its individual components. In addition, the issue of core-valence electron correlation separation is explored; it is found that approximate additive treatments of the two effects have limitations that are significant in the realm of1 kJ mol(-1) theoretical thermochemistry.

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), 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.

Advanced ab initio [coupled cluster theory through quasiperturbative triple excitations (CCSD(T))] and density functional (B3LYP) computational chemistry approaches were used in combination with the standard and augmented correlation consistent polarized valence basis sets [cc-pVnZ and aug-cc-pVnZ, where n=D(2), T(3), Q(4), and 5] to investigate the energetic and structural properties of small molecules containing third-row (Ga-Kr) atoms. These molecules were taken from the Gaussian-2 (G2) extended test set for third-row atoms. Several different schemes were used to extrapolate the calculated energies to the complete basis set (CBS) limit for CCSD(T) and the Kohn-Sham (KS) limit for B3LYP. Zero point energy and spin orbital corrections were included in the results. Overall, CCSD(T) atomization energies, ionization energies, proton affinities, and electron affinities are in good agreement with experiment, within 1.1 kcal/mol when the CBS limit has been determined using a series of two basis sets of at least triple zeta quality. For B3LYP, the overall mean absolute deviation from experiment for the three properties and the series of molecules is more significant at the KS limit, within 2.3 and 2.6 kcal/mol for the cc-pVnZ and aug-cc-pVnZ basis set series, respectively.

The accuracy of density functional theory in the description of geometries and atomization energies has been assessed by comparison to experimental data for a series of first-row closed-shell molecules. Six commonly used functionals (B3LYP, B3PW91, B3P86, BLYP, BPW91, BP86) were investigated in combination with the correlation-consistent basis sets [cc-pVxZ and aug-cc-pVxZ where x=D(2), T(3), Q(4), 5]. The convergence of molecular properties with respect to increasing basis set size has been examined. A full statistical error analysis has been performed, assessing the success of each functional with respect to each basis set in terms of both accuracy and precision. Overall, there is smooth convergence towards the Kohn-Sham limit for the hybrid functionals B3LYP and B3PW91, whereas the nonlocal functionals are relatively insensitive to basis set choice.