Showing papers on "Coupled cluster published in 2010"

A Computational Investigation of Organic Dyes for Dye-Sensitized Solar Cells: Benchmark, Strategies, and Open Issues

Abstract: A comprehensive theoretical study on the electronic absorption spectra of a representative group of organic dyes (L0, D4, D5, C217, and JK2) employed in dye-sensitized solar cell devices is reported. A benchmark evaluation on different time-dependent density functional theory (TDDFT) approaches with respect to high-level correlated coupled cluster (CC) and multireference perturbation theory (MRPT) benchmark calculations is performed in the gas phase. The benchmark results indicate that TDDFT calculations using the hybrid MPW1K and the long-range correct CAM-B3LYP functionals represent a valuable tool of comparable accuracy to that of the much more computationally demanding ab initio methods. Thus, the problem of the comparison between the calculated excitation energies and the measured absorption maximum wavelengths has been addressed employing the MPW1K functional and including the solvation effects by a polarizable continuum model. The present results show that taking into account the chemical and physi...

Assessment of TD-DFT methods and of various spin scaled CIS(D) and CC2 versions for the treatment of low-lying valence excitations of large organic dyes

Abstract: We present an extension of our previously published benchmark set for low-lying valence transitions of large organic dyes [L. Goerigk et al., Phys. Chem. Chem. Phys.11, 4611 (2009)]. The new set comprises in total 12 molecules, including two charged species and one with a clear charge-transfer transition. Our previous study on TD-DFT methods is repeated for the new test set with a larger basis set. Additionally, we want to shed light on different spin-scaled variants of the configuration interaction singles with perturbative doubles correction [CIS(D)] and the approximate coupled cluster singles and doubles method (CC2). Particularly for CIS(D) we want to clarify, which of the proposed versions can be recommended. Our results indicate that an unpublished SCS-CIS(D) variant, which is implemented into the TURBOMOLE program package, shows worse results than the original CIS(D) method, while other modified versions perform better. An SCS-CIS(D) version with a parameterization, that has already been used in an application by us recently [L. Goerigk and S. Grimme, ChemPhysChem9, 2467 (2008)], yields the best results. Another SCS-CIS(D) version and the SOS-CIS(D) method [Y. M. Rhee and M. Head-Gordon, J. Phys. Chem. A111, 5314 (2007)] perform very similar, though. For the electronic transitions considered herein, there is no improvement observed when going from the original CC2 to the SCS-CC2 method but further adjustment of the latter seems to be beneficial. Double-hybrid density functionals belong to best methods tested here. Particularly B2GP-PLYP provides uniformly good results for the complete set and is considered to be close to chemical accuracy within an ab initiotheory of color. For conventional hybrid functionals, a Fock-exchange mixing parameter of about 0.4 seems to be optimum in TD-DFT treatments of large chromophores. A range-separated functional such as, e.g., CAM-B3LYP seems also to be promising.

Communications: Accurate and efficient approximations to explicitly correlated coupled-cluster singles and doubles, CCSD-F12

Abstract: We propose a novel explicitly correlated coupled-cluster singles and doubles method CCSD(F12∗), which retains the accuracy of CCSD-F12 while the computational costs are only insignificantly larger than those for a conventional CCSD calculation.

Benchmarks of electronically excited states: basis set effects on CASPT2 results.

Abstract: Vertical excitation energies and one-electron properties are computed for the valence excited states of 28 medium-sized organic benchmark molecules using multistate multiconfigurational second-order perturbation theory (MS-CASPT2) and the augmented correlation-consistent aug-cc-pVTZ basis set. They are compared with previously reported MS-CASPT2 results obtained with the smaller TZVP basis. The basis set extension from TZVP to aug-cc-pVTZ causes rather minor and systematic shifts in the vertical excitation energies that are normally slightly reduced (on average by 0.11 eV for the singlets and by 0.09 eV for the triplets), whereas the changes in the calculated oscillator strengths and dipole moments are somewhat more pronounced on a relative scale. These basis set effects at the MS-CASPT2 level are qualitatively and quantitatively similar to those found at the coupled cluster level for the same set of benchmark molecules. The previously proposed theoretical best estimates (TBE-1) for the vertical excitation energies for 104 singlet and 63 triplet excited states of the benchmark molecules are upgraded by replacing TZVP with aug-cc-pVTZ data that yields a new reference set (TBE-2). Statistical evaluations of the performance of density functional theory (DFT) and semiempirical methods lead to the same ranking and very similar quantitative results for TBE-1 and TBE-2, with slightly better performance measures with respect to TBE-2. DFT/MRCI is most accurate among the investigated DFT-based approaches, while the OMx methods with orthogonalization corrections perform best at the semiempirical level.

The UV absorption of nucleobases: semi-classical ab initio spectra simulations

Abstract: Semi-classical simulations of the UV-photoabsorption cross sections of adenine, guanine, cytosine, thymine, and uracil in gas phase were performed at the resolution-of-identity coupled cluster to the second-order (RI-CC2) level. With the exception of cytosine, the spectra of the other four nucleobases show a two band pattern separated by a low intensity region. The spectrum of cytosine is shaped by a sequence of three bands of increasing intensity. The first band of guanine is composed by two ππ* transitions of similar intensities. The analysis of individual contributions to the spectra allows a detailed assignment of bands. It is shown that the semi-classical simulations are able to predict general features of the experimental spectra, including their absolute intensities.

Molecular core-valence correlation effects involving the post-d elements Ga-Rn: benchmarks and new pseudopotential-based correlation consistent basis sets.

Abstract: Correlation consistent basis sets that are suitable for the correlation of the outer-core (n−1)spd electrons of the post-d elements Ga–Rn have been developed. These new sets, denoted by cc-pwCVXZ-PP (X=D,T,Q,5), are based on the previously reported cc-pVXZ-PP sets that were built in conjunction with accurate small-core relativistic pseudopotentials (PPs) and designed only for valence nsp correlation. These new basis sets have been utilized in benchmark coupled cluster calculations of the core-valence correlation effects on the dissociation energies and spectroscopic properties of several small molecules. As expected, the most important contribution is the correlation of the (n−1)d electrons. For example, in the case of the group 13 homonuclear diatomics (Ga2,In2,Tl2), this leads to a dissociation energy increase compared to a valence-only treatment from 1.5 to 3.2 kcal/mol, bond length shortenings from −0.076 to −0.125 A, and harmonic frequency increases of 7–8 cm−1. Even in the group 15 cases (As2,Sb2,Bi2), the analogous effects of (n−1)d electron correlation are certainly not insignificant, the largest values being +4.4 kcal/mol, −0.049 A, and +9.6 cm−1 for the effects on De, re, and ωe, respectively. In general, the effects increase in magnitude down a group from 4p to 6p. Correlation of the outer-core (n−1)p electrons is about an order of magnitude less important than (n−1)d but larger than that of the (n−1)s. The effect of additional tight functions for Hartree–Fock and valence sp correlation was found to be surprisingly large, especially for the post-4d and post-5d elements. The pseudopotential results for the molecules containing post-3d elements are also compared to the analogous all-electron calculations employing the Douglas–Kroll–Hess Hamiltonian. The errors attributed to the PP approximation are found to be very small.

Ab initio coupled-cluster approach to nuclear structure with modern nucleon-nucleon interactions

Abstract: We perform coupled-cluster calculations for the doubly magic nuclei $^{4}\mathrm{He}$, $^{16}\mathrm{O}$, $^{40,48}\mathrm{Ca}$, for neutron-rich isotopes of oxygen and fluorine, and employ ``bare'' and secondary renormalized nucleon-nucleon interactions. For the nucleon-nucleon interaction from chiral effective field theory at order next-to-next-to-next-to leading order, we find that the coupled-cluster approximation including triples corrections binds nuclei within 0.4 MeV per nucleon compared to data. We employ interactions from a resolution-scale dependent similarity renormalization group transformations and assess the validity of power counting estimates in medium-mass nuclei. We find that the missing contributions from three-nucleon forces are consistent with these estimates. For the unitary correlator model potential, we find a slow convergence with respect to increasing the size of the model space. For the $G$-matrix approach, we find a weak dependence of ground-state energies on the starting energy combined with a rather slow convergence with respect to increasing model spaces. We also analyze the center-of-mass problem and present a practical and efficient solution.

Linear scaling coupled cluster method with correlation energy based error control.

Abstract: Coupled cluster calculations can be carried out for large molecular systems via a set of calculations that use small orbital fragments of the full molecular orbital space. The error in the correlation energy of the full molecular system is controlled by the precision in the small fragment calculations. The determination of the orbital spaces for the small orbital fragments is black box in the sense that it does not depend on any user—provided molecular fragmentation, rather orbital spaces are carefully selected and extended during the calculation to give fragment energies of a specified precision. The computational method scales linearly with the size of the molecular system and is massively parallel.

RESEARCH ARTICLE Explicitly correlated coupled cluster methods with pair-specific geminals

Abstract: Explicitly correlated MP2-F12 and CCSD(T)-F12 methods with orbital-pair-specific Slatertype geminals are proposed. The fixed amplitude ansatz of Ten-no is used, and different exponents of the Slater geminal functions can be chosen for core-core, core-valence, and valencevalence pairs. This takes care of the different sizes of the correlation hole and leads to improved results when inner-shell orbitals are correlated. The complications and the extra computational cost as compared to corresponding calculations with a single geminal are minor. The improved accuracy of the method is demonstrated for spectroscopic properties of Br2, As2, Ga2, Cu2, GaCl, CuCl, and CuBr, where the d-orbitals are treated as core.

Kinetics of hydrogen-transfer isomerizations of butoxyl radicals

Abstract: Five isomerization reactions involving intramolecular hydrogen-transfer in butoxyl radicals have been studied using variational transition state theory with small curvature tunneling. A set of best estimates of barrier heights and reaction energies for these five reactions was obtained by using coupled cluster theory including single and double excitations with a quasiperturbative treatment of connected triple excitations and a basis set extrapolated to the complete basis set limit plus core–valence correlation contributions and scalar relativistic corrections. This work predicts high-pressure limiting rate constants of these five reactions over the temperature range 200–2500 K and clarifies the available experimental data from indirect measurements. This study shows the importance of performing rate calculations with proper accounting for tunneling and torsional anharmonicity. We also proposed two new models for use in fitting rate constants over wide ranges of temperature.

Coumarin Dyes for Dye-Sensitized Solar Cells - A Long-Range-Corrected Density Functional Study

Abstract: The excited-state properties in a series of coumarin solar cell dyes are investigated with a long-range-corrected (LC) functional which asymptotically incorporates Hartree-Fock exchange. Using time-dependent density functional theory (TDDFT), we calculate excitation energies, oscillator strengths, and excited-state dipole moments in each of the dyes as a function of the range-separation paramenter, mu. To investigate the acceptable range of mu and assess the quality of the LC-TDDFT formalism, an extensive comparison is made between LC-BLYP excitation energies and approximate coupled cluster singles and doubles (CC2) calculations. When using a properly-optimized value of mu, we find that the LC technique provides a consistent picture of charge-transfer excitations as a function of molecular size. In contrast, we find that the widely-used B3LYP hybrid functional severely overestimates excited-state dipole moments and underestimates vertical excitations energies, especially for larger dye molecules. The results of the present study emphasize the importance of long-range exchange corrections in TDDFT for investigating the excited-state properties in solar cell dyes.

Reaction mechanism of monoethanolamine with CO2 in aqueous solution from molecular modeling

Abstract: We present a theoretical study of the reaction mechanism of monoethanolamine (MEA) with CO2 in an aqueous solution. We have used molecular orbital reaction pathway calculations to compute reaction free energy landscapes for the reaction steps involved in the formation of carbamic acids and carbamates. We have used the conductor-like polarizable continuum model to calculate reactant, product, and transition state geometries and vibrational frequencies within density functional theory (DFT). We have also computed single point energies for all stationary structures using a coupled cluster approach with singles, doubles, and perturbational triple excitations using the DFT geometries. Our calculations indicate that a two-step reaction mechanism that proceeds via a zwitterion intermediate to form carbamate is the most favorable reaction channel. The first step, leading to formation of the zwitterion, is found to be rate-determining, and the activation free energies are 12.0 (10.2) and 11.3 (9.6) kcal/mol using ...

Photoisomerization of Model Retinal Chromophores: Insight from Quantum Monte Carlo and Multiconfigurational Perturbation Theory

Abstract: We present a systematic investigation of the structural relaxation in the excited state of model retinal chromophores in the gas phase using the complete-active-space self-consistent theory (CASSCF), multiconfigurational second-order perturbation theory (CASPT2), quantum Monte Carlo (QMC), and coupled cluster (CC) methods. In contrast to the CASSCF photoisomerization mechanism of bond inversion followed by torsion around formal double bonds, we find that the other approaches predict an initial skeletal relaxation which does not lead to bond inversion but to a rather flexible retinal chromophore with longer bonds and with the bond-length pattern of the ground state being partly preserved. The relaxation proceeds then preferentially via partial torsion around formal single bonds and does not reach a conical intersection region. Our findings are compatible with solution experiments which point to the existence of multiple minima and relaxation pathways, some of which are nonreactive, do not lead to photoproducts via conical intersection, and are dominant in solution. Our results also demonstrate the importance of a balanced description of dynamical and static correlation in the excited-state gradients and raise serious concerns on the common use of the CASSCF method to investigate structural properties of photoexcited retinal systems.

Quantum chemical modeling of benzene ethylation over H-ZSM-5 approaching chemical accuracy: a hybrid MP2:DFT study.

Abstract: The alkylation of benzene by ethene over H-ZSM-5 is analyzed by means of a hybrid MP2:DFT scheme Density functional calculations applying periodic boundary conditions (PBE functional) are combined with MP2 energy calculations on a series of cluster models of increasing size which allows extrapolation to the periodic MP2 limit Basis set truncation errors are estimated by extrapolation of the MP2 energy to the complete basis set limit Contributions from higher-order correlation effects are accounted for by CCSD(T) coupled cluster calculations The sum of all contributions provides the “final estimates” for adsorption energies and energy barriers Dispersion contributes significantly to the potential energy surface As a result, the MP2:DFT potential energy profile is shifted downward compared to the PBE profile More importantly, this shift is not the same for reactants and transition structures due to different self-interaction correction errors The final enthalpies for ethene, benzene, and ethylbenzen

Basis set effects on coupled cluster benchmarks of electronically excited states: CC3, CCSDR(3) and CC2

Abstract: Vertical electronic excitation energies and one-electron properties of 28 medium-sized molecules from a previously proposed benchmark set are revisited using the augmented correlation-consistent triple-zeta aug-cc-pVTZ basis set in CC2, CCSDR(3), and CC3 calculations The results are compared to those obtained previously with the smaller TZVP basis set For each of the three coupled cluster methods, a correlation coefficient greater than 0994 is found between the vertical excitation energies computed with the two basis sets The deviations of the CC2 and CCSDR(3) results from the CC3 reference values are very similar for both basis sets, thus confirming previous conclusions on the intrinsic accuracy of CC2 and CCSDR(3) This similarity justifies the use of CC2- or CCSDR(3)-based corrections to account for basis set incompleteness in CC3 studies of vertical excitation energies For oscillator strengths and excited-state dipole moments, CC2 calculations with the aug-cc-pVTZ and TZVP basis sets give correla

Solvation of the excited states of chromophores in polarizable environment: orbital relaxation versus polarization.

Abstract: A hybrid quantum mechanics/molecular mechanics (QM/MM) method for the electronic excited states has been developed. The equation-of-motion coupled cluster with single and double excitations method (EOM-CCSD) is used for the QM region, while the effective fragment potential (EFP) method describes a MM part. The EFP method overcomes the most significant limitation of QM/MM by replacing empirical MM interactions and QM/MM coupling by parameter-free first-principles-based ones, while retaining the computational efficiency of QM/MM. The developed QM/MM scheme involves quantum-mechanical coupling of the electrostatic and polarization terms in the QM/MM Hamiltonian and allows accurate calculation of the electronic excited states of chromophores in various environments. Applications to the water complexes of formaldehyde and p-nitroaniline show that the orbital relaxation of the solute in the electric field of the solvent provides the majority of the solvatochromic effect, and the response of the polarizable envi...

Benchmark studies of variational, unitary and extended coupled cluster methods

Abstract: Comparative benchmark calculations are presented for coupled cluster theory in its standard formulation, as well as variational, extended, and unitary coupled cluster methods. The systems studied include HF, N2, and CN, and with cluster operators that for the first time include up to quadruple excitations. In cases where static correlation effects are weak, the differences between the predictions of molecular properties from each theory are negligible. When, however, static correlation is strong, it is demonstrated that variational coupled cluster theory can be significantly more robust than the traditional ansatz and offers a starting point on which to base single-determinant reference methods that can be used beyond the normal domain of applicability. These conclusions hold at all levels of truncation of the cluster operator, with the variational approach showing significantly smaller errors.

Random phase approximation correlation energies with exact Kohn-Sham exchange

Abstract: The random phase approximation (RPA) correlation energy is expressed in terms of the exact local Kohn–Sham (KS) exchange potential and corresponding adiabatic and nonadiabatic exchange kernels for density-functional reference determinants. The approach naturally extends the RPA method in which, conventionally, only the Coulomb kernel is included. By comparison with the coupled cluster singles doubles with perturbative triples method it is shown for a set of small molecules that the new RPA method based on KS exchange yields correlation energies more accurate than RPA on the basis of Hartree–Fock exchange.

Comparison of aromatic NH···π, OH···π, and CH···π interactions of alanine using MP2, CCSD, and DFT methods.

Abstract: A comparison of the performance of various density functional methods including long-range corrected and dispersion corrected methods [MPW1PW91, B3LYP, B3PW91, B97-D, B1B95, MPWB1K, M06-2X, SVWN5, ωB97XD, long-range correction (LC)-ωPBE, and CAM-B3LYP using 6-31+G(d,p) basis set] in the study of CH···π, OH···π, and NH···π interactions were done using weak complexes of neutral (A) and cationic (A+) forms of alanine with benzene by taking the Moller–Plesset (MP2)/6-31+G(d,p) results as the reference. Further, the binding energies of the neutral alanine–benzene complexes were assessed at coupled cluster (CCSD)/6-31G(d,p) method. Analysis of the molecular geometries and interaction energies at density functional theory (DFT), MP2, CCSD methods and CCSD(T) single point level reveal that MP2 is the best overall performer for noncovalent interactions giving accuracy close to CCSD method. MPWB1K fared better in interaction energy calculations than other DFT methods. In the case of M06-2X, SVWN5, and the dispersion corrected B97-D, the interaction energies are significantly overrated for neutral systems compared to other methods. However, for cationic systems, B97-D yields structures and interaction energies similar to MP2 and MPWB1K methods. Among the long-range corrected methods, LC-ωPBE and CAM-B3LYP methods show close agreement with MP2 values while ωB97XD energies are notably higher than MP2 values. © 2010 Wiley Periodicals, Inc. J Comput Chem 2010

Full implementation and benchmark studies of Mukherjee's state-specific multireference coupled-cluster ansatz

Abstract: The state-specific multireference coupled-cluster (SS-MRCC) ansatz developed by Mukherjee and co-workers [J. Chem. Phys. 110, 6171 (1999)] has been implemented by means of string-based techniques. The implementation is general and allows for using arbitrary complete active spaces of any spin multiplicity and arbitrarily high excitations in the cluster operators. Several test calculations have been performed for single- and multiple-bond dissociations of molecular systems. Our experience shows that convergence problems are encountered when solving the working equations of the SS-MRCC in the case the weight of one or more reference functions tends to take on very small values. This is system specific and cannot yet be handled in a black-box fashion. The problem can be obviated by either dropping all the cluster amplitudes from the corresponding model functions with coefficients below a threshold or by a regularization procedure suggested by Tikhonov or a combination of both. In the current formulation the S...

The coupled-cluster revolution

Abstract: The elements of coupled-cluster (CC) theory that distinguish it from other ways of treating the correlation problem are presented. These essential components required that new theory be developed to treat analytical gradients, excited states, higher order properties, and now multi-reference CC. Aided by these developments, CC theory provides the best answers for the largest number of problems in molecular structure and spectra. Many scientists who spent time at the Quantum Theory Project were instrumental in these developments.

Frozen natural orbitals for ionized states within equation-of-motion coupled-cluster formalism.

Abstract: The frozen natural orbital (FNO) approach, which has been successfully used in ground-state coupled-cluster calculations, is extended to open-shell ionized electronic states within equation-of-motion coupled-cluster (EOM-IP-CC) formalism. FNOs enable truncation of the virtual orbital space significantly reducing the computational cost with a negligible decline in accuracy. Implementation of the MP2-based FNO truncation scheme within EOM-IP-CC is presented and benchmarked using ionized states of beryllium, dihydrogen dimer, water, water dimer, nitrogen, and uracil dimer. The results show that the natural occupation threshold, i.e., percentage of the total natural occupation recovered in the truncated virtual orbital space, provides a more robust truncation criterion as compared to the fixed percentage of virtual orbitals retained. Employing 99%-99.5% natural occupation threshold, which results in the virtual space reduction by 70%-30%, yields errors below 1 kcal/mol. Moreover, the total energies exhibit linear dependence as a function of the percentage of the natural occupation retained allowing for extrapolation to the full virtual space values. The capabilities of the new method are demonstrated by the calculation of the 12 lowest vertical ionization energies (IEs) and the lowest adiabatic IE of guanine. In addition to IE calculations, we present the scans of potential energy surfaces (PESs) for ionized (H(2)O)(2) and (H(2))(2). The scans demonstrate that the FNO truncation does not introduce significant nonparallelity errors and accurately describes the PESs shapes and the corresponding energy differences, e.g., dissociation energies.

A review of canonical transformation theory

Abstract: Canonical transformation (CT) theory targets the description of dynamic correlation in multireference quantum chemistry problems. When combined with a static correlation quantum chemistry method, it enables the quantitative description of chemical processes involving electronic structure not described by a single electronic configuration. We argue that many multireference dynamic correlation methods display unsatisfactory characteristics, including lack of size-consistency, a low-order treatment of correlation, and a poor computational scaling. By contrast, CT theory is based on an exponential ansatz that is rigorously size-consistent, reduces in a single-reference limit to a coupled cluster theory, and has an n^6 computational scaling with system and active space size. The efficient formulation of CT theory has allowed it to be applied to difficult systems in conjunction with active spaces with more than 30 orbitals, beyond the reach of traditional methods, with an accuracy that far exceeds multireference perturbation theories. Here we review the basic motivation, formulation, and implementation of CT theory, as well as survey some of our recent applications and possible future directions.

Multilevel extension of the cluster-in-molecule local correlation methodology: merging coupled-cluster and Møller-Plesset perturbation theories.

Abstract: A multilevel extension of the local correlation “cluster-in-molecule” (CIM) framework, which enables one to combine different quantum chemistry methods to treat different regions in a large molecul...

Perturbative triples corrections in state-specific multireference coupled cluster theory.

Abstract: We formulated and implemented a perturbative triples correction for the state-specific multireference coupled cluster approach with singles and doubles suggested by Mukherjee and co-workers, Mk-MRCCSD [Mol. Phys. 94, 157 (1998)]. Our derivation of the energy correction [Mk-MRCCSD(T)] is based on a constrained search for stationary points of the Mk-MRCC energy functional together with a perturbative expansion with respect to the appearing triples cluster operator. The Λ-Mk-MRCCSD(T) approach derived in this way consists in (1) a correction to the off-diagonal matrix elements of the effective Hamiltonian which is unique to coupled cluster methods based on the Jeziorski–Monkhorst ansatz, and (2) an asymmetric energy correction to the diagonal elements of the effective Hamiltonian. The Mk-MRCCSD(T) correction is obtained from the Λ-Mk-MRCCSD(T) method by approximating the singles and doubles Lagrange multipliers with the corresponding cluster amplitudes. We investigate the performance of the Mk-MRCCSD(T) meth...

Systematic study of first-row transition-metal diatomic molecules: a self-consistent DFT+U approach.

Abstract: We present a systematic first-principles study of the equilibrium bond lengths, harmonic frequencies, dissociation energies, ground state symmetries, and spin state splittings of 22 diatomic molecules comprised of a first-row 3d transition-metal and a main-group element (H, C, N, O, or F). Diatomic molecules are building blocks of the key molecular bonding motifs in biological and inorganic catalytic systems, but, at the same time, their small size permits a thorough study by even the most computationally expensive quantum chemistry approaches. The results of several density-functional theory (DFT) approaches including hybrid, generalized-gradient, and generalized-gradient augmented with Hubbard U exchange-correlation functionals are presented. We compare these efficiently calculated DFT results with the highly accurate but computationally expensive post-Hartree-Fock approaches multireference configuration interaction (MRCI) and coupled cluster [CCSD(T)] as well as experimental values, where available. We show that by employing a Hubbard U approach, we systematically reduce average errors in state splittings and dissociation energies by a factor of 3. We are also able to reassign the ground state of four molecules improperly identified by hybrid or generalized-gradient approaches and provide correct assignment of all ground state symmetries as compared against experimental assignment and MRCI reference. By providing accuracy comparable to more expensive quantum chemistry approaches with the robust scaling of the generalized-gradient approximation, our DFT+U approach permits the study of very large scale systems with vastly improved results.

Improved design of orbital domains within the cluster-in-molecule local correlation framework: single-environment cluster-in-molecule ansatz and its application to local coupled-cluster approach with singles and doubles.

Abstract: The improved variant of the local correlation coupled-cluster (CC) framework termed "cluster-in-molecule" (CIM), defining the single-environment (SE) CIM-CC approach, is presented and tested at the CC singles and doubles (CCSD) level. In the proposed SECIM-CC method, the previous design of the CIM orbital subsystems [Li, W.; Gour, J. R.; Piecuch, P.; Li, S. J. Chem. Phys. 2009, 131, 114109], referred to as the dual-environment (DE) CIM-CC approach, which is based on the ideas of central orbitals and the associated primary and secondary environments, is replaced by the simplified design in which the central localized molecular orbitals (LMOs) and the corresponding environment LMOs are first assigned to each nonhydrogen atom and the hydrogen atoms that are bound to it. The SECIM-CC approach offers improvements in the DECIM-CC results, particularly for weakly bound molecular clusters using diffuse basis functions. Through the use of a single parameter to define the environment LMOs and through the assignment of subsystem LMOs to atoms, the SECIM-CC calculations are easy to control and the CIM subsystems do not unnecessarily vary with the nuclear geometry, creating smoother potential energy surfaces. The performance of SECIM-CCSD is illustrated by the calculations for normal alkanes and water clusters described by the 6-31G(d), 6-31++G(d,p), and 6-311++G(d,p) basis sets.

Threshold photoelectron spectroscopy of the methyl radical isotopomers, CH3, CH2D, CHD2 and CD3: synergy between VUV synchrotron radiation experiments and explicitly correlated coupled cluster calculations.

Abstract: Threshold photoelectron spectra (TPES) of the isotopomers of the methyl radical (CH3, CH2D, CHD2, and CD3) have been recorded in the 9.5−10.5 eV VUV photon energy range using third generation synchrotron radiation to investigate the vibrational spectroscopy of the corresponding cations at a 7−11 meV resolution. A threshold photoelectron−photoion coincidence (TPEPICO) spectrometer based on velocity map imaging and Wiley−McLaren time-of-flight has been used to simultaneously record the TPES of several radical species produced in a Ar-seeded beam by dc flash-pyrolysis of nitromethane (CHxDyNO2, x + y = 3). Vibrational bands belonging to the symmetric stretching and out-of-plane bending modes have been observed and P, Q, and R branches have been identified in the analysis of the rotational profiles. Vibrational configuration interaction (VCI), in conjunction with near-equilibrium potential energy surfaces calculated by the explicitly correlated coupled cluster method CCSD(T*)-F12a, is used to calculate vibrat...

Stochastic coupled cluster theory.

Abstract: We describe a stochastic coupled cluster theory which represents excitation amplitudes as discrete excitors in the space of excitation amplitudes. Reexpressing the coupled cluster (CC) equations as the dynamics of excitors in this space, we show that a simple set of rules suffices to evolve a distribution of excitors to sample the CC solution and correctly evaluate the CC energy. These rules are not truncation specific and this method can calculate CC solutions to an arbitrary level of truncation. We present results of calculation on the neon atom, and nitrogen and water molecules showing the ability to recover both truncated and full CC results.