Showing papers on "Coupled cluster published in 2003"

Systematically convergent basis sets with relativistic pseudopotentials. II. Small-core pseudopotentials and correlation consistent basis sets for the post-d group 16–18 elements

Abstract: A series of correlation consistent basis sets have been developed for the post-d group 16–18 elements in conjunction with small-core relativistic pseudopotentials of the energy-consistent variety. The latter were adjusted to multiconfiguration Dirac–Hartree–Fock data based on the Dirac–Coulomb–Breit Hamiltonian. The outer-core (n−1)spd shells are explicitly treated together with the nsp valence shell with these PPs. The accompanying cc-pVnZ-PP and aug-cc-pVnZ-PP basis sets range in size from DZ to 5Z quality and yield systematic convergence of both Hartree–Fock and correlated total energies. In addition to the calculation of atomic electron affinities and dipole polarizabilities of the rare gas atoms, numerous molecular benchmark calculations (HBr, HI, HAt, Br2, I2, At2, SiSe, SiTe, SiPo, KrH+, XeH+, and RnH+) are also reported at the coupled cluster level of theory. For the purposes of comparison, all-electron calculations using the Douglas–Kroll–Hess Hamiltonian have also been carried out for the haloge...

Systematically convergent basis sets with relativistic pseudopotentials. I. Correlation consistent basis sets for the post-d group 13–15 elements

Abstract: New correlation consistent-like basis sets have been developed for the post-d group 13–15 elements (Ga–As, In–Sb, Tl–Bi) employing accurate, small-core relativistic pseudopotentials. The resulting basis sets, which are denoted cc-pVnZ-PP, are appropriate for valence electron correlation and range in size from (8s7p7d)/[4s3p2d] for the cc-pVDZ-PP to (16s13p12d3f2g1h)/[7s7p5d3f2g1h] for the cc-pV5Z-PP sets. Benchmark calculations on selected diatomic molecules (As2, Sb2, Bi2, AsN, SbN, BiN, GeO, SnO, PbO, GaCl, InCl, TlCl, GaH, InH, and TlH) are reported using these new basis sets at the coupled cluster level of theory. Much like their all-electron counterparts, the cc-pVnZ-PP basis sets yield systematic convergence of total energies and spectroscopic constants. In several cases all-electron benchmark calculations were also carried out for comparison. The results from the pseudopotential and all-electron calculations were nearly identical when scalar relativity was accurately included in the all-electron wo...

Geometry optimizations with the coupled-cluster model CC2 using the resolution-of-the-identity approximation

Abstract: An implementation of the gradient for the second-order coupled-cluster singles-and-doubles model CC2 is reported, which employs the resolution-of-the-identity (RI) approximation for electron repulsion integrals. The performance of the CC2 model for ground state equilibrium geometries and harmonic frequencies is investigated and compared with experiment and other ab initio methods. It is found that CC2 equilibrium geometries have a similar accuracy to those calculated with second-order Moller–Plesset perturbation theory (MP2), but the bond lengths are larger. In particular, double and triple bonds and bonds in electron-rich compounds are elongated by 0.5–1.5 pm. Thereby CC2 slightly outperforms MP2 for single bonds, in particular in electron-rich compounds, but for strong double and triple bonds CC2 is somewhat inferior to MP2. The results for harmonic frequencies go in parallel with the results for equilibrium structures. The error introduced by the RI approximation is found to be negligible compared to t...

Analytic gradients for excited states in the coupled-cluster model CC2 employing the resolution-of-the-identity approximation

Abstract: The derivation and implementation of excited state gradients is reported for the approximate coupled-cluster singles and doubles model CC2 employing the resolution-of-the-identity approximation for electron repulsion integrals. The implementation is profiled for a set of examples with up to 1348 basis functions and exhibits no I/O bottlenecks. A test set of sample molecules is used to assess the performance of the CC2 model for adiabatic excitation energies, excited state structure constants and vibrational frequencies. We find very promising results, especially for adiabatic excitation energies, though the need of a single-reference ground state and a single-replacement dominated excited state puts some limits on the applicability of the method. Its reliability, however, can always be tested on grounds of diagnostic measures. As an example application, we present calculations on the π*←π excited state of trans-azobenzene.

Geometrical and electronic structures of gold, silver, and gold-silver binary clusters: Origins of ductility of gold and gold-silver alloy formation

Abstract: The structures of pure gold and silver clusters (Auk, Agk, k = 1−13) and neutral and anionic gold−silver binary clusters (AumAgn, 2 ≤ k = m + n ≤ 7) have been investigated by using density functional theory (DFT) with generalized gradient approximation (GGA) and high level ab initio calculations including coupled cluster theory with relativistic ab initio pseudopotentials. Pure Auk clusters favor 2-D planar configurations, while pure Agk clusters favor 3-D structures. In the case of Au, the valence orbital energies of 5d are close to that of 6s. This allows the hybridization of 6s and 5d orbitals in favor of planar structures of Auk clusters. Even 1-D linear structures show reasonable stability as local minima (or as global minima in a few small anionic clusters). This explains the ductility of gold. On the other hand, the Ag-4d orbital has a much lower energy than the 5s. This prevents hybridization, and so the coordination number (Nc) of Ag in Agk tends to be large in s-like spherical 3-D coordination i...

Local treatment of electron excitations in the EOM-CCSD method

Abstract: The Equation-of-Motion coupled cluster method restricted to single and double excitations (EOM-CCSD) and singlet excited states is formulated in a basis of nonorthogonal local orbitals. In the calculation of excited states only electron promotions from localized molecular orbitals into subspaces (excitation domains) of the local basis are allowed, which strongly reduces the number of EOM-CCSD amplitudes to be optimized. Furthermore, double excitations are neglected unless the excitation domains of the corresponding localized occupied orbitals are close to each other. Unlike in the local methods for the ground state, the excitation domains cannot be simply restricted to the atomic orbitals that are spatially close to the localized occupied orbitals. In the present paper the choice of the excitation domains is based on the analysis of wave functions computed by more approximate (and cheaper) methods like, e.g., configuration-interaction singles. The effect of various local approximations is investigated in detail, and it is found that a balanced description of the local configuration spaces describing the ground and excited states is essential to obtain accurate results. Using a single set of parameters for a given basis set, test calculations with the local EOM-CCSD method were performed for 14 molecules and 49 electronically excited states. The excitation energies computed by the local EOM-CCSD method reproduce the conventional EOM-CCSD excitation energies with an average error of 0.06 eV.

Performance of coupled cluster theory in thermochemical calculations of small halogenated compounds

Abstract: Atomization energies at 0 K and heats of formation at 298 K were obtained for a collection of small halogenated molecules from coupled cluster theory including noniterative, quasiperturbative triple excitations calculations with large basis sets (up through augmented septuple zeta quality in some cases). In order to achieve near chemical accuracy (±1 kcal/mol) in the thermodynamic properties, we adopted a composite theoretical approach which incorporated estimated complete basis set binding energies based on frozen core coupled cluster theory energies and (up to) five corrections: (1) a core/valence correction; (2) a Douglas–Kroll–Hess scalar relativistic correction; (3) a first-order atomic spin–orbit correction; (4) a second-order spin–orbit correction for heavy elements; and (5) an approximate correction to account for the remaining correlation energy. The last of these corrections is based on a recently proposed approximation to full configuration interaction via a continued fraction approximant for coupled cluster theory [CCSD(T)-cf]. Failure to consider corrections (1) to (4) can introduce errors significantly in excess of the target accuracy of ±1 kcal/mol. Although some cancellation of error may occur if one or more of these corrections is omitted, such a situation is by no means universal and cannot be relied upon for high accuracy. The accuracy of the Douglas–Kroll–Hess approach was calibrated against both new and previously published four-component Dirac Coulomb results at the coupled cluster level of theory. In addition, vibrational zero-point energies were computed at the coupled cluster level of theory for those polyatomic systems lacking an experimental anharmonic value.

Analytic second derivatives for general coupled-cluster and configuration-interaction models.

Abstract: Analytic second derivatives of energy for general coupled-cluster (CC) and configuration-interaction (CI) methods have been implemented using string-based many-body algorithms. Wave functions truncated at an arbitrary excitation level are considered. The presented method is applied to the calculation of CC and CI harmonic frequencies and nuclear magnetic resonance chemical shifts up to the full CI level for some selected systems. The present benchmarks underline the importance of higher excitations in high-accuracy calculations.

Continuous transition between Brillouin-Wigner and Rayleigh-Schrödinger perturbation theory, generalized Bloch equation, and Hilbert space multireference coupled cluster

Abstract: A continuous transition between the Rayleigh–Schrodinger and Brillouin–Wigner perturbation theories is constructed and the Bloch equation for the corresponding wave operator is derived. Subsequently it is applied to the Hilbert space multireference coupled cluster theory and used to investigate relationships between several versions of multireference coupled cluster methods. Finally, based on those continuous transitions, new size extensivity corrections for the Brillouin–Wigner coupled cluster method are suggested. Numerical tests of size-extensivity and separability of a supermolecule to closed- and open-shell fragments are also presented. Equivalence of some of the multireference coupled cluster methods with single and double excitations to full configuration interaction for two-electron systems is investigated, both theoretically and numerically.

Equation-of-motion coupled cluster method with full inclusion of the connected triple excitations for ionized states: IP-EOM-CCSDT

Abstract: The equation-of-motion (EOM) coupled cluster (CC) method with full inclusion of the connected triple excitations for ionization energies has been formulated and implemented. Using proper factorization of the three- and four-body parts of the effective Hamiltonian, an efficient computational procedure has been proposed for IP-EOM-CCSDT which at the EOM level requires no-higher-than nocc3nvir4 scaling. The method is calibrated by the evaluation of the valence vertical ionization potentials for CO, N2, and F2 molecules for several basis sets up to 160 basis functions. At the basis set limit, errors vary from 0.0 to 0.2 eV, compared to “experimental” vertical ionization potentials.

Equation-of-motion coupled cluster method with full inclusion of connected triple excitations for electron-attached states: EA-EOM-CCSDT

Abstract: We extend the full triples equation-of-motion (EOM) coupled cluster (CC) method to electron attached states. Proper factorization of the three- and four-body parts of the effective Hamiltonian makes it possible to achieve for the EA-EOM part a scaling no higher than nocc2nvir5. The method is calibrated by the evaluation of the valence vertical electron affinities for the C2 and O3 molecules for several basis sets up to 160 basis functions. For C2, EA-EOM-CCSDT gives 3.24 eV at the extrapolated basis limit, while the experimental adiabatic EA is equal to 3.27±0.008 eV. For O3 the agreement is ∼1.9 eV compared to an adiabatic value of 2.1 eV.

Accurate electric multipole moment, static polarizability and hyperpolarizability derivatives for N2

Abstract: We report accurate values of the electric moments, static polarizabilities, hyperpolarizabilities and their respective derivatives for N2. Our values have been extracted from finite-field Moller–Pleset perturbation theory and coupled cluster calculations performed with carefully designed basis sets. A large [15s12p9d7f] basis set consisting of 290 CGTF is expected to provide reference self-consistent-field values of near-Hartree–Fock quality for all properties. The Hartree–Fock limit for the mean hyperpolarizability is estimated at γ=715±4e4a04Eh−3 at the experimental bond length Re=2.074 32a0. Accurate estimates of the electron correlation effects were obtained with a [10s7p6d4f] basis set. Our best values are Θ=−1.1258ea02 for the quadrupole and Φ=−6.75ea04 for the hexadecapole moment, ᾱ=11.7709 and Δα=4.6074e2a02Eh−1 for the mean and the anisotropy of the dipole polarizability, C=41.63e2a04Eh−1 for the mean quadrupole polarizability and γ=927e4a04Eh−3 for the dipole hyperpolarizability. The latter v...

Exact solution (within a triple-zeta, double polarization basis set) of the electronic Schrödinger equation for water

Abstract: Using a newly developed density matrix renormalization group algorithm, we have computed exact solutions of the Schrodinger equation for water at two geometries in a basis of 41 orbitals. Calculations of this size cannot be carried out using any other method. We compare our results with high-order coupled cluster and configuration interaction calculations.

Linear response functions for coupled cluster/molecular mechanics including polarization interactions

Abstract: We present the first implementation of linear response theory for the coupled cluster/molecular mechanics (CC/MM) method. This model introduces polarization effects into a quantum mechanical/molecular mechanical (QM/MM) framework using a self-consistent procedure while electrostatic effects are modeled by assigning partial charges to the MM molecules and a van der Waals potential describes dispersion and short range repulsion. The quantum mechanical subsystem is described using coupled cluster electronic structure methods. The response theory for the calculation of molecular properties for such a model is described and implemented at the coupled cluster singles and doubles (CCSD) level. Sample calculations of excitation energies, transition moments and frequency dependent polarizabilities for liquid water are presented. Finally, we consider the development of a parameter independent iterative self-consistent CC/MM model where the properties calculated by CC/MM response theory are used in the QM/MM interaction Hamiltonian.

The interaction of oxygen with small gold clusters

Abstract: Presented in this work are the results of a quantum chemical study of oxygen adsorption on small Aun and Aun− (n=2,3) clusters. Density functional theory (DFT), second order perturbation theory (MP2), and singles and doubles coupled cluster theory with perturbative triples [CCSD(T)] methods have been used to determine the geometry and the binding energy of oxygen to Aun. The multireference character of the wave functions has been studied using the complete active space self-consistent field method. There is considerable disagreement between the oxygen binding energies provided by CCSD(T) calculations and those obtained with DFT. The disagreement is often qualitative, with DFT predicting strong bonds where CCSD(T) predicts no bonds or structures that are bonded but have energies that exceed those of the separated components. The CCSD(T) results are consistent with experimental measurements, while DFT calculations show, at best, a qualitative agreement. Finally, the lack of a regular pattern in the size and...

Benchmark theoretical study of the ionization threshold of benzene and oligoacenes

Abstract: In straightforward continuation of Green’s function studies of the ultraviolet photoelectron spectra of polycyclic aromatic compounds [Deleuze et al., J. Chem. Phys. 115, 5859 (2001); M. S. Deleuze, ibid. 116, 7012 (2002)], we present a benchmark theoretical determination of the ionization thresholds of benzene, naphthalene, anthracene, naphthacene (tetracene), pentacene, and hexacene, within chemical accuracy [0.02–0.07 eV]. The vertical ionization potentials of these compounds have been obtained from series of single-point calculations at the Hartree–Fock, second-, third-, and partial fourth-order Moller–Plesset (MP2, MP3, MP4SDQ) levels, and from coupled cluster calculations including single and double excitations (CCSD) as well as a perturbative estimate of connected triple excitations [CCSD(T)], using basis sets of improving quality, introducing up to 510, 790, 1070, 1350, 1630, and 1910 basis functions in the computations, respectively. A focal point analysis of the convergence of the calculated ionization potentials has been performed in order to extrapolate the CCSD(T) results to an asymptotically (cc-pV∞Z) complete basis set. The present results confirm the adequacy of the outer-valence Green’s function scheme for strongly correlated systems. Adiabatic ionization energies have been further determined by incorporating Beck-three-parameter Lee–Yang–Parr functional corrections for zero-point vibrational energies and for geometrical relaxations. Extension of the analysis to the CCSD(T)/cc-pV∞Z level shows that the energy minimum form of the benzene radical cation is an obtuse structure related to the 2B2g state. Isotopic shifts of the adiabatic ionization potentials, due to deuterium substitution of hydrogens, have also been discussed.

Fundamental world of quantum chemistry : a tribute to the memory of Per-Olov Löwdin

Abstract: Per-Olov Lowdin.- In SilicoChemistry: Past, Present and Future.- Weights of Spin and Permutational Symmetry Adapted States for Arbitrary Elementary Spins.- Schrodinger's Wave Equation - A Lie Algebra Treatment.- On Supersymmetric Quantum Mechanics.- Application of Lowdin's Metric Matrix: Atomic Spectral Methods for Electronic Structure Calculations.- Integrals for Exponentially Correlated Four-Body Systems of General Angular Symmetry.- Appendix to "Studies in Perturbation Theory": The Problem of Partitioning.- Treating Nonadditivity as a Perturbation: a Quasi-Particle Formalism.- Unified Approach to Intensities in Vibrational Spectroscopies via Dynamic Electromagnetic Shieldings at the Nuclei of a Molecule.- Comparison Between the Many-Body Perturbative and Green's-Function Approaches for Calculating Electron Binding Energies and Affinities: Brueckner and Dyson Orbitals.- Quantum Chemistry, Localization, Superconductivity, and Mott-Hubbard U.- Reformulation of the Concept of Jahn-Teller Vibronic Coupling Effects in Theoretical Chemistry.- Collisions of Atoms and Molecules in External Magnetic Fields.- Effects of Orbital Overlap on Calculations of Charge Exchange in Atom-Surface Scattering.- Relativistic Quantum Chemistry of Heavy and Superheavy Elements: Fully Relativistic Coupled-Cluster Calculations for Molecules of Heavy and Transactinide Superheavy Elements.- Study of Heavy Elements by Relativistic Fock Space and Intermediate Hamiltonian Coupled Cluster Methods.- QED Effects in Heavy Elements.- Time and Time Reversal Symmetry in Quantum Chemical Kinetics.- Solute-Solvent Ineractions from QM SCRF Methods, Analysis of Group Contributions to Solvation.- The Cavity Model with a Surface Formed by Two Intersecting Spheres. An Analytical Treatment.- Quantum Mechanical Calculations on Molecules Containing Positrons.- Low-Lying Ionization Potentials of B3N and Photodetachment Energies of B3N- Using The Multiconfigurational Spin Tensor Electron Propagator Method.- Understanding Alkyl Substituent Effects in R-O Bond Dissociation Reactions in Open- and Closed-Shell Systems.- Hydrogen Bonding and the Energetics of Homolytic Dissociation in Solution.- Theoretical Calculations of Kinetic Isotope Effects for a Series of Substituted Aziridines.- Exploring The Catalytic Cycle of the Hydrosylilation of Alkenes Catalyzed by Hydrido-Bridged Diplatinum Complexes Using Electronic Structure Calculation Methods.- Towards A Physical Explanation of the Periodic Table (PT) of Chemical Elements.

Basis set and correlation dependent extrapolation of correlation energy

Abstract: A simple extrapolation formula of (X+γ)−3 which fits correlation energies with correlation consistent (aug-)cc-pVXZ and (aug-)cc-pV(X+1)Z[X=D(2),T(3),Q(4)] basis sets to estimate the basis set limit was devised by varying the parameter γ according to basis set quality and correlation level. The explicit extrapolation formulas suitable for calculations at the second order Moller–Plesset perturbation theory and single and double excitation coupled cluster theory with perturbative triples correction level are presented and applications are made to estimate the basis set limit binding energies of various hydrogen-bonded and van der Waals clusters. A comparison of the results by this formula with the reference basis set limit results and the results by other extrapolation methods reveals that the extrapolation formulas proposed here can yield the reliable basis set limit estimates even with the small basis sets and could be used effectively for investigating large weakly bound complexes.

Accuracy of geometries: Influence of basis set, exchange-correlation potential, inclusion of core electrons, and relativistic corrections

Abstract: The geometries of a set of small molecules were optimized using eight different exchange–correlation (xc) potentials in a few different basis sets of Slater-type orbitals, ranging from a minimal basis (I) to a triple-zeta valence basis plus double polarization functions (VII). This enables a comparison of the accuracy of the xc potentials in a certain basis set, which can be related to the accuracies of wavefunction-based methods such as Hartree–Fock and coupled cluster. Four different checks are done on the accuracy by looking at the mean error, standard deviation, mean absolute error and maximum error. It is shown that the mean absolute error decreases with increasing basis set size, and reaches a basis set limit at basis VI. With this basis set, the mean absolute errors of the xc potentials are of the order of 0.7–1.3 pm. This is comparable to the accuracy obtained with CCSD and MP2/MP3 methods, but is still larger than the accuracy of the CCSD(T) method (0.2 pm). The best performing xc potentials are found to be Becke–Perdew, PBE and PW91, which perform as well as the hybrid B3LYP potential. In the second part of this paper, we report the optimization of the geometries of five metallocenes with the same potentials and basis sets, either in a nonrelativistic or a scalar relativistic calculation using the zeroth-order regular approximation approach. For the first-row transition-metal complexes, the relativistic corrections have a negligible effect on the optimized structures, but for ruthenocene they improve the optimized Ru–ring distance by some 1.4–2.2 pm. In the largest basis set used, the absolute mean error is again of the order of 1.0 pm. As the wavefunction-based methods either give a poor performance for metallocenes (Hartree–Fock, MP2), or the size of the system makes a treatment with accurate methods such as CCSD(T) in a reasonable basis set cumbersome, the good performance of density functional theory calculations for these molecules is very promising; even more so as density functional theory is an efficient method that can be used without problems on systems of this size, or larger.

Many-body effects in nonadiabatic molecular theory for simultaneous determination of nuclear and electronic wave functions: Ab initio NOMO/MBPT and CC methods

Abstract: We have investigated the many-body effects in a molecular theory to determine simultaneously nuclear and electronic wave functions without the Born–Oppenheimer (BO) approximation. We first apply the many-body perturbation theory using the electron–nucleus and nucleus–nucleus interactions to the non-BO theory and show the importance of the electron–nucleus correlation rather than the nucleus–nucleus one. We next combine the non-BO theory with the coupled cluster double and Brueckner double methods using the one-electron plus one-nucleus excitation operators.

A discrete solvent reaction field model within density functional theory

Abstract: In this work we present theory and implementation for a discrete reaction field model within Density Functional Theory (DFT) for studying solvent effects on molecules. The model combines a quantum mechanical (QM) description of the solute and a classical description of the solvent molecules (MM). The solvent molecules are modeled by point charges representing the permanent electronic charge distribution, and distributed polarizabilities for describing the solvent polarization arising from many-body interactions. The QM/MM interactions are introduced into the Kohn–Sham equations, thereby allowing for the solute to be polarized by the solvent and vice versa. Here we present some initial results for water in aqueous solution. It is found that the inclusion of solvent polarization is essential for an accurate description of dipole and quadrupole moments in the liquid phase. We find a very good agreement between the liquid phase dipole and quadrupole moments obtained using the Local Density Approximation and results obtained with a similar model at the Coupled Cluster Singles and Doubles level of theory using the same water cluster structure. The influence of basis set and exchange correlation functional on the liquid phase properties was investigated and indicates that for an accurate description of the liquid phase properties using DFT a good description of the gas phase dipole moment and molecular polarizability are also needed.

Substituent Effects on Scalar 2J(19F,19F) and 3J(19F,19F) NMR Couplings: A Comparison of SOPPA and DFT Methods

Abstract: Substituent effects for 2J(F,F) couplings in aliphatic and olefinic CF2 moieties and 3J(F,F) couplings in fluorinated derivatives of ethylene were studied using both high level ab initio and DFT/B3LYP calculations. Where possible, J variations have been compared with experimental values. In general, the SOPPA (second-order polarization propagator approximation) methodology matches absolute experimental values reasonably well, whereas the DFT/B3LYP approach performs poorly in describing 2J(F,F) couplings. Fortunately, substituent effects for DFT J couplings are notably better reproduced. For a vinyl CF2 moiety, the accurate prediction of 2J(F,F) couplings is a challenging task even for high level ab initio methods such as SOPPA and SOPPA(CCSD) (second-order polarization propagator approximation with coupled cluster singles and doubles amplitudes). Aliphatic 2J(F,F) couplings are very sensitive to the electronegativity of substituents placed α to the CF2 group. The latter J perturbations are dominated large...

Mercury Monoxide: A Systematic Investigation of Its Ground Electronic State

Abstract: One of the possible key products in the oxidation and transport of atmospheric mercury is mercury monoxide, HgO. In the present work, large-scale multireference configuration interaction, as well as coupled cluster, wave functions have been used in conjunction with a series of correlation consistent basis sets to calculate the near-equilibrium potential energy functions of the first two electronic states of HgO. In the absence of spin−orbit coupling, the lowest 1Σ+ and 3Π states are nearly isoenergetic with a binding energy with respect to ground-state atoms of only 2−3 kcal/mol at the estimated complete basis set limit. After spin−orbit coupling effects are accurately included, the resulting X0+ state has a D0 of just 4.0 kcal/mol, leading to a ΔHf(0 K) of 70.4 kcal/mol. These values are in stark contrast to the currently accepted experimental values. The implications of these results to atmospheric mercury chemistry are briefly discussed.

Critical comparison of single-reference and multireference coupled-cluster methods: Geometry, harmonic frequencies, and excitation energies of N2O2

Abstract: Calculated vertical excitation energies, optimized geometries, and vibrational frequencies of the nitric oxide dimer are reported. The “multireference” (MR) nature of the problem and weak bond between the monomers make a proper description of the system difficult, and standard methods are not as applicable to this system. In this study, recently developed methods such as the double-electron-affinity similarity-transformed equation-of-motion coupled cluster method (DEA-STEOM-CCSD), MR Brillouin–Wigner CCSD (MR-BWCCSD), MR average quadratic CCSD (MR-AQCCSD), and others are used along with a series of basis sets of increasing accuracy. The calculated excitation energies are consistent and convergent with respect to the basis set in DEA-STEOM-CCSD, MR-BWCCSD, and MR-AQCCSD methods. The geometries are highly sensitive to the basis set size and the challenge to obtain the right answers in the basis set limit remains. Nevertheless, we obtain qualitative agreement with the experimental geometry and harmonic vibra...

Theoretical investigation of weak hydrogen bonds to sulfur

Abstract: The interaction energies of the dimethylsulfide–methanol (I) and dimethylthiocarbonyl–methanol (II) complexes are calculated as a function of the S⋯H–O distances at various levels of theory and compared to those of their oxygen analogs. At the coupled cluster level the binding energy of (I) is −5.46 kcal/mol, only slightly smaller than the hydrogen bond energy of −5.97 kcal/mol for the corresponding oxygen analog, i.e., the dimethylether–methanol complex. It is also considerably larger than for dimethylether–methylthiol, where S and O of the parent complex are interchanged. Density functional theory is unable to describe these weak interactions properly. Choosing second-order Moller–Plesset perturbation theory, the interaction potential surfaces of both complexes with respect to the three relevant intermolecular coordinates are compared. The interactions in the hydrogen bonds involving sulfur are classified by Morokuma, atoms-in-molecules, and natural bond orbital analyses.

The standard enthalpy of formation of CH2

Abstract: High-quality ab initio quantum chemical methods, including higher-order coupled cluster and full configuration interaction benchmarks, with basis sets ranging from [C/H] [4s3p1d/2s1p] to [9s8p7d5f4g3h2i/7s6p5d4f3g2h] have been employed to obtain the best technically possible value for the standard enthalpy of formation of X 3B1 CH2 and a 1A1 CH2. Careful extrapolations of finite basis MP2, CCSD, CCSD(T), and CCSDT energies to the complete basis set full configuration interaction limit plus inclusion of small corrections owing to relativistic effects, core correlation, and the diagonal Born–Oppenheimer correction results in the final extrapolated enthalpies of formation of this study, ΔfH0o(X 3B1 CH2)=390.45−0.64+0.68 kJ mol−1 and ΔfH0o(a 1A1 CH2)=428.10−0.64+0.68 kJ mol−1. The computed value for X 3B1 CH2 is in between the best two experimental results of 389.87±0.86 and 390.73±0.66 kJ mol−1. The elaborate calculations leading to these enthalpies of formation also resulted in accurate estimates of the singlet-triplet splitting, T0(a 1A1 CH2)=37.54−0.29+0.41 kJ mol−1, in excellent agreement with the best empirical value of 37.65±0.06 kJ mol−1, of the total atomization enthalpy, D0(X 3B1 CH2)=753.03−0.62+0.43 kJ mol−1, in excellent agreement with the best experimental value of 753.3 kJ mol−1, of the bond dissociation energy, DU1o(0 K)(CH–H)=417.85±0.35 kJ mol−1, and of the quartic force field representations of the potentials of the two states around their respective minima.

Coupled Cluster Theory and Multireference Configuration Interaction Study of FO, F2O, FO2, and FOOF

Abstract: Structures, vibrational frequencies, atomization energies at 0 K, and heats of formation at 298 K were obtained for four oxyfluoride molecules, several of which are known to present difficulties for single reference ab initio methods. Whereas much of this work was carried out with coupled cluster theory, multireference configuration interaction calculations were also performed, as an independent check on the reliability of the former. The use of large basis sets (up through augmented sextuple zeta quality in some cases) and a simple basis set extrapolation formula enabled us to accurately estimate the complete basis set limit. However, to achieve near chemical accuracy (±1 kcal/mol) in the thermodynamic properties, it was necessary to include three corrections to the frozen core atomization energies, in addition to the zero-point vibrational energy: (1) a core/valence correction; (2) a Douglas−Kroll−Hess scalar relativistic correction; and (3) a first-order atomic spin−orbit correction. Several approache...

Automated calculation of fundamental frequencies: Application to AlH3 using the coupled-cluster singles-and-doubles with perturbative triples method

Abstract: An automated scheme for calculating numerical derivatives of functions is presented and applied to the Taylor expansion of potential energy surfaces. The computational cost is reduced by invoking the symmetry properties of noncubic groups. The scheme is applied to the quartic force field of isotopomers of AlH3 by numerical differentiation of the CCSD~T! energy, using the cc-pCVQZ basis for the harmonic part of the potential and the cc-pCVTZ basis for the anharmonic part. From this force field, zero-order vibrational corrections to the geometry and the fundamental frequencies are calculated by second-order perturbation theory. The results are compared with experiment and previous calculations. © 2003 American Institute of Physics. @DOI: 10.1063/1.1583671#