Showing papers on "Coupled cluster published in 1999"

Towards standard methods for benchmark quality ab initio thermochemistry :w1 and w2 theory

Abstract: Two new schemes for computing molecular total atomization energies (TAEs) and/or heats of formation (ΔHf∘) of first- and second-row compounds to very high accuracy are presented. The more affordable scheme, W1 (Weizmann-1) theory, yields a mean absolute error of 0.30 kcal/mol and includes only a single, molecule-independent, empirical parameter. It requires CCSD (coupled cluster with all single and double substitutions) calculations in spdf and spdfg basis sets, while CCSD(T) (i.e., CCSD with a quasiperturbative treatment of connected triple excitations) calculations are only required in spd and spdf basis sets. On workstation computers and using conventional coupled cluster algorithms, systems as large as benzene can be treated, while larger systems are feasible using direct coupled cluster methods. The more rigorous scheme, W2 (Weizmann-2) theory, contains no empirical parameters at all and yields a mean absolute error of 0.23 kcal/mol, which is lowered to 0.18 kcal/mol for molecules dominated by dynami...

Ab initio quantum chemistry using the density matrix renormalization group

Abstract: In this paper we describe how the density matrix renormalization group can be used for quantum chemical calculations for molecules, as an alternative to traditional methods, such as configuration interaction or coupled cluster approaches. As a demonstration of the potential of this approach, we present results for the H2O molecule in a standard gaussian basis. Results for the total energy of the system compare favorably with the best traditional quantum chemical methods.

A size-consistent state-specific multireference coupled cluster theory: Formal developments and molecular applications

Abstract: In this paper we present a comprehensive account of a manifestly size-consistent coupled cluster formalism for a specific state, which is based on a reference function composed of determinants spanning a complete active space (CAS). The method treats all the reference determinants on the same footing and is hence expected to provide uniform description over a wide range of molecular geometry. The combining coefficients are determined by diagonalizing an effective operator in the CAS and are thus completely flexible, not constrained to preassigned values. A separate exponential-type excitation operator is invoked to induce excitations to all the virtual functions from each reference determinant. The linear dependence inherent in this choice of cluster operators is eliminated by invoking suitable sufficiency conditions, which in a transparent manner leads to manifest size extensivity. The use of a CAS also guarantees size consistency. We also discuss the relation of our method with the extant state-specific...

Ground state potential energy curves for He2, Ne2, Ar2, He–Ne, He–Ar, and Ne–Ar: A coupled-cluster study

Abstract: Potential energy curves for three homonuclear (He2, Ne2, Ar2) and three heteronuclear (He–Ne, He–Ar, Ne–Ar) rare gas dimers are presented. The curves were calculated using several correlation consistent basis sets and the supermolecule single and double excitation coupled-cluster theory with noniterative perturbational treatment of triple excitations, CCSD(T). The most accurate results were obtained with the aug-cc-pV5Z basis set supplemented with an additional (3s3p2d2f1g) set of bond functions. The results obtained with a smaller aug-cc-pVQZ+(3s3p2d2f1g) basis set are almost as accurate. Both basis sets give results in better agreement with potentials based on experiments than the recent results obtained with larger d-aug-cc-pV6Z and t-aug-cc-pV6Z basis sets but without bond functions. For each complex and each basis set a fitted potential energy curve is given. In addition, for each complex, with the exception of He2, the values of Re, De, B0, D0, and 〈R〉0 are given. For He2 no bound states were found ...

Linear scaling coupled cluster and perturbation theories in the atomic orbital basis

Abstract: We present a reformulation of the coupled cluster equations in the atomic orbital (AO) basis that leads to a linear scaling algorithm for large molecules. Neglecting excitation amplitudes in a screening process designed to achieve a target energy accuracy, we obtain an AO coupled cluster method which is competitive in terms of number of amplitudes with the traditional molecular orbital (MO) solution, even for small molecules. For large molecules, the decay properties of integrals and excitation amplitudes becomes evident and our AO method yields a linear scaling algorithm with respect to molecular size. We present benchmark calculations to demonstrate that our AO reformulation of the many-body electron correlation problem defeats the “exponential scaling wall” that has characterized high-level MO quantum chemistry calculations for many years.

Re-examination of atomization energies for the Gaussian-2 set of molecules

Abstract: Atomization energies were computed for 73 molecules, many of them chosen from the GAUSSIAN-2 and G2/97 test sets. A composite theoretical approach was adopted which incorporated estimated complete basis set binding energies based on frozen core coupled cluster theory with a quasiperturbative treatment of triple excitations and three corrections: (1) a coupled cluster core/valence correction; (2) a configuration interaction scalar relativistic correction; and (3) an atomic spin-orbital correction. A fourth correction, corresponding to more extensive correlation recovery via coupled cluster theory with an approximate treatment of quadruple excitations, was examined in a limited number of cases. For the molecules and basis sets considered in this study, failure to consider any of these contributions to the atomization energy can introduce errors on the order of 1–2 kcal/mol. Although some cancellation of error is common, it is by no means universal and cannot be relied upon for high accuracy. With the largest available basis sets (including, in some cases, up through aug-cc-pV6Z), the mean absolute deviation with respect to experiment was found to lie in the 0.7–0.8 kcal/mol range, neglecting the effects of higher order excitations. Worst case errors were 2–3 kcal/mol. Several complete basis set extrapolations were tested with regard to their effectiveness at improving agreement with experiment, but the statistical difference among the various approaches was small.

A Critical Validation of Density Functional and Coupled-Cluster Approaches for the Calculation of EPR Hyperfine Coupling Constants in Transition Metal Complexes

Abstract: The performance of various density functional approaches for the calculation of electron paramagnetic resonance (EPR) hyperfine coupling constants in transition metal complexes has been evaluated critically by comparison with experimental data and high-level coupled-cluster results for 21 systems, representing a large variety of different electronic situations. While both gradient-corrected and hybrid functionals allow the calculation of isotropic metal hyperfine coupling constants to within ca. 10−15% for the less critical cases (e.g., ScO, TiN, TiO, VO, MnO, MnF), none of the functionals investigated performs well for all complexes. Gradient-corrected functionals tend to underestimate the important core−shell spin polarization. While this may be improved by exact-exchange mixing in some cases, the accompanying spin contamination may even lead to a deterioration of the results for other complexes. We also identify cases, where essentially none of the functionals performs satisfactorily. In the absence of...

Benchmark calculations with correlated molecular wavefunctions. XIII. Potential energy curves for He2, Ne2 and Ar2 using correlation consistent basis sets through augmented sextuple zeta

Abstract: The potential energy curves of the rare gas dimers He2, Ne2, and Ar2 have been computed using correlation consistent basis sets ranging from singly augmented aug-cc-pVDZ sets through triply augmented t-aug-cc-pV6Z sets, with the augmented sextuple basis sets being reported herein. Several methods for including electron correlation were investigated, namely Moller—Plesset perturbation theory (MP2, MP3 and MP4) and coupled cluster theory [CCSD and CCSD(T)]. For He2 CCSD(T)/d-aug-cc-pV6Z calculations yield a well depth of 7.35 cm-1 (10.58 K), with an estimated complete basis set (CBS) limit of 7.40 cm-1 (10.65 K). The latter is smaller than the 'exact' well depth (Aziz, R. A., Janzen, A. R., and Moldover, M. R., 1995, Phys. Rev. Lett., 74, 1586) by about 0.2 cm-1 (0.35 K). The Ne2 well depth, computed with the CCSD(T)/d-aug-cc-pV6Z method, is 28.31 cm-1 and the estimated CBS limit is 28.4 cm-1, approximately 1 cm-1 smaller than the empirical potential of Aziz, R. A., and Slaman, M., J., 1989, Chem. Phys., 13...

Predicting the binding energies of H-bonded complexes: A comparative DFT study

Abstract: Comparisons with the results of coupled cluster calculations were made to assess the quality of density functionals in predicting the electronic binding energies of H-bonded complexes. A variety of different density functionals, namely B3LYP, B97-1, PBE0, HCTH, BLYP, PBE, LDA and a recently derived improvement of the HCTH functional (HCTH38), as well as the standard abinitio Hartree–Fock and second-order Moller–Plesset perturbation theory methods were applied using a triple-ζ plus double polarisation basis set. Equilibrium structures, counterpoise corrected binding energies and harmonic frequencies were calculated for the (HF)2, (HCl)2, (H2O)2, (CO)(HF), (OC)(HF), (FH)(NH3), (ClH)(NH3), (H2O)(NH3) and (H3O+)(H2O) complexes. Although the hybrid methods performed well in general, the new HCTH38 functional as a pure GGA predicted binding energies of better quality than the B3LYP functional. Bond length changes and frequency shifts were compared to MP2 results.

Assessment of the single-root multireference Brillouin–Wigner coupled- cluster method: Test calculations on CH2, SiH2, and twisted ethylene

Abstract: Recently developed single-root multireference Brillouin–Wigner coupled-cluster (MR BWCC) theory, which belongs to a broad family of state-selective multireference coupled-cluster methods, has been implemented in the ACES II program package at the CCSD level of approximation. The method represents a new approach to quasidegenerate problems, which is able to continuously switch between the single-reference CC in a nondegenerate situation and the Hilbert-space MRCC in a degenerate case. An assessment of the method has been carried out by means of a comparison with the full configuration interaction (CI) treatments of CH2, SiH2, and twisted ethylene diradicals. The problem of size-extensivity is discussed.

A simple scheme for the direct calculation of ionization potentials with coupled-cluster theory that exploits established excitation energy methods

Abstract: Vertical ionization potentials can be obtained from existing computer programs for the high-level treatment of excited states by simply including a continuum orbital in the basis set. Exploiting this feature easily allows final state energies for ionized states to be calculated at several previously untested levels of theory that go beyond the equation-of-motion coupled-cluster singles and doubles model. Values obtained for N2, CO, and F2 with the most theoretically complete approximations studied here (those based on the CCSDT-3 and CC3 parametrizations of the neutral ground state) are in excellent agreement with experiment when a large basis set is used.

Infinite basis limits in electronic structure theory

Abstract: We have developed a database of 29 molecules for which we have estimated the complete-one-electron-basis-set limit of the zero-point-exclusive atomization energy for five levels of electronic structure theory: Hartree–Fock (HF) theory, Mo/ller–Plesset second- and fourth-order perturbation theory, coupled cluster theory based on single and double excitations (CCSD), and CCSD plus a quasiperturbative treatment of triple excitations [CCSD(T)], all at a single set of standard geometries. Convergence checks indicate that the estimates are within a few tenths of a kcal/mol of the n=infinity limit of the cc-pVnZ basis set sequence. This data is then used to obtain optimized power-law exponents for extrapolating to the basis-set-limit from correlation-consistent polarized valence double and triple zeta (cc-pVDZ and cc-pVTZ) basis sets. This allows one to get thermochemical accuracy comparable to polarized quadruple or quintuple zeta (cc-pVQZ or cc-pV5Z) basis sets with a cost very comparable to polarized triple z...

Heats of Formation of Simple Perfluorinated Carbon Compounds

Abstract: The heats of formation of CF3, its cation and anion, CF4, C2F4, and :CFCF3 have been calculated at high levels of ab initio molecular orbital theory. Geometries and frequencies were determined, in general, with second-order perturbation theory. Total energies based on coupled cluster calculations with perturbative triples were determined with basis sets up through augmented quadruple-ζ in quality and were subsequently extrapolated to the complete one-particle basis set limit, so as to further reduce the basis set truncation error. Due to its importance as a standard, CF4 was studied with even larger basis sets. Additional improvements in the atomization energy were achieved by applying corrections due to core/valence correlation, scalar relativistic, and atomic spin−orbit effects. Zero-point energies were based on the experimental fundamentals, when available, and harmonic frequencies obtained from MP2/cc-pVTZ calculations. Missing frequencies for :CFCF3 were calculated at the MP2/DZP level. The calculate...

Effects of basis set and electron correlation on the calculated interaction energies of hydrogen bonding complexes: MP2/cc-pV5Z calculations of H2O–MeOH,H2O–Me2O,H2O–H2CO, MeOH–MeOH, and HCOOH–HCOOH complexes

Abstract: The MP2 intermolecular interaction energies of the title complexes were calculated with the Dunning’s correlation consistent basis sets (cc-pVXZ, X=D, T, Q, and 5) and the interaction energies at the basis set limit were estimated. The second-order Mo/ller–Plesset (MP2) interaction energies greatly depend on the basis sets used, while the Hartree–Fock (HF) energies do not. Small basis sets considerably underestimate the attractive interaction. The coupled cluster single double triple [CCSD(T)] interaction energies are close to the MP2 ones. The expected CCSD(T) interaction energies of the H2O–MeOH, H2O–Me2O, H2O–H2CO, MeOH–MeOH, and HCOOH–HCOOH complexes at the basis set limit are −4.90, −5.51, −5.17, −5.45, and −13.93 kcal/mol, respectively, while the HF/cc-pV5Z energies are −3.15, −2.58, −3.60, −2.69, and −11.29 kcal/mol, respectively. The HF calculations greatly underestimate the attractive energies and fail to predict the order of the bonding energies in these complexes. These results show that a large basis set and the consideration of an appropriate electron correlation correction are essential to study interactions of hydrogen bonding complexes by ab initio molecular orbital calculation.

Basis-set convergence of the molecular electric dipole moment

Abstract: The electric dipole moments (μ) of BH and HF are computed in conventional calculations employing different correlation-consistent basis sets at the levels of Hartree–Fock theory, second-order perturbation theory, and coupled cluster theory with single and double excitations, and single and double excitations with a perturbative triples correction. The basis-set convergence of μ is examined by comparison with results obtained with explicitly correlated wave function models. Inclusion of diffuse functions in the basis set is essential for accurate calculations of μ. They speed up the convergence at the Hartree–Fock level significantly and make the convergence at the correlated levels systematic. Once the outer valence regions important for μ are described accurately via the diffuse functions, the convergence at the correlated levels is governed by the interelectronic Coulomb singularity. For the aug-cc-pVXZ basis sets, the correlation contribution to μ follows μXcorr=μlimcorr+aX−3, which is similar to the f...

On the accurate theoretical determination of the static hyperpolarizability of trans-butadiene

Abstract: Finite-field many-body perturbation theory and coupled cluster calculations are reported for the static second dipole hyperpolarizability γαβγδ of trans-butadiene. A very large basis set of [9s6p4d1f/6s3p1d] size (336 contracted Gaussian-type functions) should lead to self-consistent field (SCF) values of near-Hartree–Fock quality. We report γxxxx=6.19, γxxxz=−0.44, γxxyy=3.42, γzzxx=2.07, γxyyz=−0.50, γxzzz=1.73, γyyyy=14.72, γyyzz=8.46, γzzzz=24.10 and γ=14.58 for 10−3×γαβγδ/e4a04Eh−3 at the experimental geometry (molecule on the xz plane with z as the main axis). γ=(14.6±0.4)×103e4a04Eh−3 should be a very reliable estimate of the Hartree–Fock limit of the mean hyperpolarizability. Keeping all other molecular geometry parameters constant, we find that near the Hartree–Fock limit the mean hyperpolarizability varies with the C=C bond length as 10−3×γ(RC=C)/e4a04Eh−3=14.93+31.78ΔR+30.88ΔR2−2.96ΔR3 and with the C–C bond length as 10−3×γ(RC–C)/e4a04Eh−3=14.93−7.20ΔR+3.04ΔR2, where ΔR/a0 is the displaceme...

EOMXCC: A New Coupled-Cluster Method for Electronic Excited States

Abstract: A new coupled-cluster (CC) theory for electronically excited, electron-attached, and ionized states of molecular systems, which takes advantages of the similarity transformation of the Hamiltonian, the equation of motion (EOM) CC method, and the expectation value CC (XCC) approach, is proposed and analyzed. The new method, which we refer to as the EOMXCC theory, is based on diagonalization of the doubly transformed Hamiltonian H ¯ ¯ = e T † H ¯ e − T † , where T represents the cluster operator and H ¯ = e − T H e T is the EOMCC Hamiltonian. In order to preserve the simple, EOMCC-like, structure of the final equations, the T cluster operators describing the ground-state problem are determined by solving the XCC equations. The new method, which is formally a single-reference approach, offers a number of advantages, which are discussed in detail. One of the important features of the EOMXCC formalism is that it seems to be capable of describing contributions from the connected triexcited clusters, which normally enter the doubles-singles block of the EOMCCSDT (EOMCC singles, doubles, and triples) eigenvalue problem, at the EOMXCCSD (EOMXCC singles and doubles) level of approximation. This finding may eventually lead to a new hierarchy of approximations, which should be capable of improving the results for the low-lying excited states dominated by double excitations from the reference, where inclusion of triples in the EOMCC scheme is essential, without using the expensive steps of the EOMCCSDT theory. The EOMXCC method is general and applies to all types of references, including RHF, UHF, and ROHF determinants. It also applies to all sectors of the Fock space. A few approximate EOMXCC schemes are considered in detail. The final equations of the EOMXCCSD theory are presented in a factorized form using recursively generated intermediates. It is demonstrated that the EOMXCC method can be viewed as a Hermitized EOMCC theory, which is capable of describing the corrections from important highorder clusters at a lower level of approximation. Along with the EOMXCC method, we discuss the possibility of generalization of the extended CC(ECC) approach to excited states. The proposed EOMECC theory uses the doubly transformed Hamiltonian H ¯ ¯ Σ = e Σ H ¯ e − Σ , where the operators T and Σ satisfy the ECC stationary conditions. The use of two independent operators T and Σ makes the EOMECC formalism even more flexible than EOMXCC, leads to a more symmetric treatment of the left- and right-hand eigenvalue problems, and to a completely exponential treatment of the left ground-state problem.

Simultaneous handling of dynamical and nondynamical correlation via reduced multireference coupled cluster method: Geometry and harmonic force field of ozone

Abstract: The geometry and vibrational frequencies of the ozone molecule are studied using the reduced multireference coupled cluster method with singles and doubles (RMR CCSD) relying on a two-electron/two-orbital (2,2) active space. The role played by the choice of the reference space is analyzed in detail by considering both the spin adapted, two-configuration reference space and the complete three-configuration reference space based on the (2,2) active space. It is shown that while the two-reference RMR CCSD approach, using the C2v symmetry-adapted (2,2) active space, provides a better description of the geometry and harmonic frequencies of symmetric vibrational modes than the standard single-reference (SR) CCSD method, the three-reference approach is essential to achieve an accurate description of the harmonic frequency of the asymmetric stretching mode. A unique behavior of ozone in this regard is highlighted by a comparison with other symmetric triatomics. The comparison with results obtained by other method...

Gauge invariant coupled cluster response theory

Abstract: We introduce a time-dependent coupled cluster based Lagrangian that includes orbital rotation. This Lagrangian is shown to give gauge invariant response properties for one-electron operators in the limit of a complete one-electron basis. The pole structure of the linear response function is compatible with that of the exact response function and the notorious problem of unphysical second-order poles in the Brueckner coupled cluster response theory is not present in this model. The total energy of the model is identical to the coupled cluster model using optimized orbitals recently revived by Sherrill et al. [J. Chem. Phys. 109, 4171 (1998)]. The model provides a straightforward approach for calculating magnetic response properties in a gauge invariant manner using coupled cluster type wave functions.

Coupled cluster response theory for solvated molecules in equilibrium and nonequilibrium solvation

Abstract: It is shown how response functions can be derived as derivatives of a time-averaged quasienergy for molecules surrounded by a polarizable dielectric continuum. The vacuum quasienergy is modified with additional solvent terms which depend on the chosen solvent configuration. We consider equilibrium and nonequilibrium solvent models. Coupled cluster response functions are derived by introducing a corresponding coupled cluster quasienergy variational Lagrangian. Computational tractable expressions are derived for linear, quadratic, and cubic response functions. Sample calculations are reported for solvation effects on excitation energies, oscillator strengths, dipole moments, and frequency-dependent polarizabilities of H2S and furan.

Electron affinities of the first- and second-row atoms: Benchmark ab initio and density-functional calculations

Abstract: A benchmark ab initio and density-functional theory (DFT) study has been carried out on the electron affinities of the first- and second-row atoms. The ab initio study involves basis sets of $\mathrm{spdfgh}$ and $\mathrm{spdfghi}$ quality, extrapolations to the one-particle basis set limit, and a combination of the coupled cluster with all single, double (and triple) excitations [CCSD(T)], CCSDT, and full configuration-interaction electron correlation methods. Scalar relativistic and spin-orbit coupling effects were taken into account. On average, the best ab initio results agree to better than 0.001 eV with the most recent experimental results. Correcting for imperfections in the CCSD(T) method improves the mean absolute error by an order of magnitude, while for accurate results on the second-row atoms inclusion of relativistic corrections is essential. The latter are significantly overestimated at the self-consistent-field level; for accurate spin-orbit splitting constants of second-row atoms, inclusion of $2s,2p$ correlation is essential. In the DFT calculations it is found that results for the first-row atoms are very sensitive to the exchange functional, while those for second-row atoms are rather more sensitive to the correlation functional. While the Lee-Yang-Parr (LYP) correlation functional works best for first-row atoms, its PW91 counterpart appears to be preferable for second-row atoms. Among ``pure DFT'' (nonhybrid) functionals, G96PW91 (Gill 1996 exchange combined with Perdew-Wang 1991 correlation) puts in the best overall performance, actually slightly better than the popular hybrid B3LYP functional. B3PW91 outperforms B3LYP, while the recently proposed one-parameter hybrid functionals such as B1LYP seem clearly superior to B3LYP and B3PW91 for first-row atoms. The best results overall are obtained with the one-parameter hybrid modified Perdew-Wang (mPW1) exchange functionals of Adamo and Barone [J. Chem. Phys. 108, 664 (1998)], with $\mathrm{mPW}1\mathrm{LY}p$ yielding the best results for first-row, and mPW1PW91 for second-row atoms. Indications are that a hybrid of the type a $\mathrm{mPW}1\mathrm{LYP}+(1\ensuremath{-}a)$ mPW1PW91 yields better results than either of the constituent functionals.

Coupled-cluster methods with internal and semi-internal triply excited clusters: Vibrational spectrum of the HF molecule

Abstract: The recently proposed, single-reference, coupled-cluster (CC) methods with singly, doubly, and triply excited clusters, in which triexcited clusters T3 are restricted to internal and semi-internal components defined through active orbitals, designated as the CCSD{t′} and CCSDt approaches [P. Piecuch, S. A. Kucharski, and R. J. Bartlett, J. Chem. Phys. 110, 6103 (1999)] have been used to obtain the potential energy function and the vibrational spectrum of the HF molecule, as described by the large, aug-cc-pvtz, basis set. A comparison has been made with the vibrational term values obtained at the very high, full CCSDT (CC singles, doubles, and triples), level and with the experimental (RKR) data. In spite of using the restricted Hartree–Fock reference, the calculated CCSD{t′} and CCSDt vibrational term values have been found to be in much better agreement with the full CCSDT and RKR data than the results of conventional CCSD (CC singles and doubles) calculations, which produce errors of an order of 2000 cm...

The MP2 limit correction applied to coupled cluster calculations of the electronic dissociation energies of the hydrogen fluoride and water dimers

Abstract: The basis set convergence of ab initio computed electronic dissociation energies is reported for the hydrogen bonded complexes (HF)2 (H2O)2. At the level of CCSD(T) theory (coupled cluster model with singles, doubles, and approximate connected triples), the interaction energy is split into one- and two-body terms, and corrections such as the counterpoise (CP) and the MP2 limit are explored. The MP2-limit correction consists of substituting the second-order Moller-Plesset (MP2) perturbation theory contribution computed with the actual basis set by the limiting value that is obtained in a complete basis. Clearly the basis set convergence of the CCSD(T) calculations is improved by the MP2 limit correction. Moreover, the MP2-limit correction can be applied irrespective of whether or not the two-body term has been CP corrected beforehand. Little difference is found between the two possibilities, but the most accurate results are obtained by applying the MP2 limit correction to CP corrected CCSD(T) two-body ter...

Integral-direct electron correlation methods

Abstract: Integral-direct implementations of Moller—Plesset perturbation theory up to fourth order (MP4(SDQ) without triple excitations), quadratic configuration interaction with single and double excitations (QCISD), coupled cluster (CCSD), multiconfiguration self-consistent field (MCSCF), second- and third-order multireference perturbation theory (MRPT2, MRPT3), and internally contracted multireference configuration interaction (MRCI) methods are described. All methods use the same three integral-direct kernels, namely direct calculation of Fock matrices, direct partial integral transformation, and direct evaluation of external exchange matrices. New algorithms for the latter two procedures are proposed and implemented, for which the memory requirement scales only linearly with basis set size. Test calculations with more than 500 basis functions are presented. It is demonstrated that integral prescreening can significantly reduce the scaling of the computational cost with molecular size N. For small polyglycine p...

Polarizabilities and first hyperpolarizabilities of HF, Ne, and BH from full configuration interaction and coupled cluster calculations

Abstract: Static and frequency-dependent polarizabilities and first hyperpolarizabilities have been calculated for HF and Ne using full configuration interaction (FCI) and a hierarchy of coupled cluster models: coupled cluster singles (CCS), an approximate coupled cluster singles and doubles model (CC2), coupled cluster singles and doubles (CCSD), an approximate coupled cluster singles, doubles, and triples model (CC3), and coupled cluster singles, doubles, and triples (CCSDT). A previous study of BH concerning FCI benchmarking has been extended to include CC3 and static CCSDT values. Systematic improvements of the polarizabilities and the hyperpolarizabilities are found going from CCS to CCSD and from CCSD to CC3 or CCSDT. Little or no improvement of the polarizabilities and no improvement of the hyperpolarizabilities are seen when going from CCS to CC2. The CCSD results represent a significant improvement over CCS and CC2 but are again surpassed by the CC3 results which agree very well with the FCI values. The re...

A coupled-cluster solvent reaction field method

Abstract: A method for introducing the coupling between a molecule and an outer solvent in molecular coupled-cluster calculations is presented. The method is a coupled-cluster self-consistent reaction field (CCSCRF) method. The outer solvent is represented as a dielectric medium which is characterized by the dielectric constants of the solvent. The interaction between solute and solvent is introduced into the coupled-cluster Lagrangian and a self-consistent solution is obtained. Sample calculations are presented for total energies and structures for H2O, H2S, and CO. Furthermore, the rotation barrier of 1,2-Dichloroethane is determined in vacuum and different solvents and the results are compared to experimental data.