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We develop a simple methodology for the computation of symmetry-adapted perturbation theory (SAPT) interaction energy contributions for intramolecular noncovalent interactions. In this approach, the local occupied orbitals of the total Hartree-Fock (HF) wavefunction are used to partition the fully interacting system into three chemically identifiable units: the noncovalent fragments A and B and a covalent linker C. Once these units are identified, the noninteracting HF wavefunctions of the fragments A and B are separately optimized while embedded in the HF wavefunction of C, providing the dressed zeroth order wavefunctions for A and B in the presence of C. Standard two-body SAPT (particularly SAPT0) is then applied between the relaxed wavefunctions for A and B. This intramolecular SAPT procedure is found to be remarkably straightforward and efficient, as evidenced by example applications ranging from diols to hexaphenyl-ethane derivatives. [ABSTRACT FROM AUTHOR]

A systematic examination of noncovalent interactions as modeled by wavefunction theory is presented in comparison to gold-standard quality benchmarks available for 345 interaction energies of 49 bimolecular complexes. Quantum chemical techniques examined include spin-component-scaling (SCS) variations on second-order perturbation theory (MP2) [SCS, SCS(N), SCS(MI)] and coupled cluster singles and doubles (CCSD) [SCS, SCS(MI)]; also, method combinations designed to improve dispersion contacts [DW-MP2, MP2C, MP2.5, DW-CCSD(T)-F12]; where available, explicitly correlated (F12) counterparts are also considered. Dunning basis sets augmented by diffuse functions are employed for all accessible Z-levels; truncations of the diffuse space are also considered. After examination of both accuracy and performance for 394 model chemistries, SCS(MI)-MP2/cc-pVQZ can be recommended for general use, having good accuracy at low cost and no ill-effects such as imbalance between hydrogen-bonding and dispersion-dominated systems or non-parallelity across dissociation curves. Moreover, when benchmarking accuracy is desirable but gold-standard computations are unaffordable, this work recommends silver-standard [DW-CCSD(T**)-F12/aug-cc-pVDZ] and bronze-standard [MP2C-F12/aug-cc-pVDZ] model chemistries, which support accuracies of 0.05 and 0.16 kcal/mol and efficiencies of 97.3 and 5.5 h for adenine · thymine, respectively. Choice comparisons of wavefunction results with the best symmetry-adapted perturbation theory [T. M. Parker, L. A. Burns, R. M. Parrish, A. G. Ryno, and C. D. Sherrill, J. Chem. Phys. 140, 094106 (2014)] and density functional theory [L. A. Burns, Á. Vázquez-Mayagoitia, B. G. Sumpter, and C. D. Sherrill, J. Chem. Phys. 134, 084107 (2011)] methods previously studied for these databases are provided for readers' guidance. [ABSTRACT FROM AUTHOR]

Symmetry-adapted perturbation theory (SAPT) offers insight into the nature of intermolecular interactions. In addition, accurate energies can be obtained from the wave function-based variant of SAPT provided that intramonomer electron correlation effects are included. We apply density-fitting (DF) approximations to the intramonomer correlation corrections in SAPT. The introduction of this approximation leads to an improvement in the computational cost of SAPT by reducing the scaling of certain SAPT terms, reducing the amount of disk I/O, and avoiding the explicit computation of certain types of MO integrals. We have implemented all the intramonomer correlation corrections to SAPT through second-order under the DF approximation. Additionally, leading third-order terms are also implemented. The accuracy of this truncation of SAPT is tested against the S22 test set of Hobza and co-workers [Phys. Chem. Chem. Phys. 8, 1985 (2006)]. When the intramonomer corrections to dispersion are included in SAPT, a mean absolute deviation of 0.3–0.4 kcal mol-1 is observed for the S22 test set when using an aug-cc-pVDZ basis. The computations on the adenine-thymine complexes in the S22 test set with an aug-cc-pVDZ basis represent the largest SAPT computations to date that include this degree of intramonomer correlation. Computations of this size can now be performed routinely with our newly developed DF-SAPT program. [ABSTRACT FROM AUTHOR]

Density fitting (DF) approximations have been used to increase the efficiency of several quantum mechanical methods. In this work, we apply DF and a related approach, Cholesky decomposition (CD), to wave function-based symmetry-adapted perturbation theory (SAPT). We also test the one-center approximation to the Cholesky decomposition. The DF and CD approximations lead to a dramatic improvement in the overall computational cost of SAPT, while introducing negligible errors. For typical target accuracies, the Cholesky basis needed is noticeably smaller than the DF basis (although the cost of constructing the Cholesky vectors is slightly greater than that of constructing the three-index DF integrals). The SAPT program developed in this work is applied to the interactions between acenes previously studied by Grimme [Angew. Chem., Int. Ed. 47, 3430 (2008)], expanding the cases studied by adding the pentacene dimer. The SAPT decomposition of the acene interactions provides a more realistic picture of the interactions than that from the energy decomposition analysis previously reported. The data suggest that parallel-displaced and T-shaped acene dimers both feature a special stabilizing π-π interaction arising from electron correlation terms which are significantly more stabilizing than expected on the basis of pairwise -C6R-6 estimates. These terms are qualitatively the same in T-shaped as in parallel-displaced geometries, although they are roughly a factor of 2 smaller in T-shaped geometries because of the larger distances between the intermolecular pairs of electrons. [ABSTRACT FROM AUTHOR]

The S22 test set of interaction energies for small model complexes [Phys. Chem. Chem. Phys. 8, 1985 (2006)] has been very valuable for benchmarking new and existing methods for noncovalent interactions. However, the basis sets utilized to compute the CCSD(T) interaction energies for some of the dimers are insufficient to obtain converged results. Here we consistently extrapolate all CCSD(T)/complete basis set (CBS) interaction energies using larger basis sets for the CCSD(T) component of the computation. The revised values, which we designate S22A, represent the most accurate results to date for this set of dimers. The new values appear to be within a few hundredths of 1 kcal mol-1 of the true CCSD(T)/CBS limit at the given geometries, but the former S22 values are off by as much as 0.6 kcal mol-1 compared to the revised values. Because some of the most promising methods for noncovalent interactions are already achieving this level of agreement (or better) compared to the S22 data, more accurate benchmark values would clearly be helpful. The MP2, SCS-MP2, SCS-CCSD, SCS(MI)-MP2, and B2PLYP-D methods have been tested against the more accurate benchmark set. The B2PLYP-D method outperforms all other methods tested here, with a mean average deviation of only 0.12 kcal mol-1. However, the consistent, slight underestimation of the interaction energies computed by the SCS-CCSD method (an overall mean absolute deviation and mean deviation of 0.24 and -0.23 kcal mol-1, respectively) suggests that the SCS-CCSD method has the potential to become even more accurate with a reoptimization of its parameters for noncovalent interactions. [ABSTRACT FROM AUTHOR]

We report on evaluation of the diagonal Born–Oppenheimer correction (DBOC) to the electronic energy with Hartree–Fock (HF) and conventional correlated wave functions for general molecular systems. Convergence of both HF and configuration interaction (CI) DBOC with the one-particle basis seems to be rather fast, with triple-ζ quality correlation consistent sets of Dunning et al. sufficiently complete to approach the respective basis set limits for the DBOC of the ground state of H[sub 2] within 0.1 cm-1. Introduction of electron correlation via the CI singles and doubles method has a substantial effect on the absolute value of the DBOC for H[sub 2], H[sub 2]O, and BH in their ground states (ca. +13 cm-1 out of 115 cm-1, +22 cm-1 out of 622 cm-1, and +11 cm-1 out of 370 cm-1, respectively). The effect of the correlation correction to the DBOC on relative energies is small, e.g., the barrier to linearity of water changes by ca. 1 cm-1; however, the value is difficult to converge to the ab initio limit. Based on recent results by Schwenke [J. Phys. Chem. A 105, 2352 (2001)] and our findings, we expect the correlation correction to the DBOC to have a substantial effect on spectroscopic properties of the ground state of water. The effect of DBOC on equilibrium bond distance r[sub e] and harmonic vibrational frequency ω[sub e] of the ground state of BH is +0.0007 Å and -2 cm-1, respectively. Surprisingly, the former is a much larger change than expected, and greater than errors due to residual incompleteness of electron correlation treatment and basis set in state-of-the-art conventional Born–Oppenheimer computations. The effect of using a correlated wave function for the DBOC evaluation on the above corrections to r[sub e] and ω[sub e] is small. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

P. Dupré, R. Jost, M. Lombardi, P. G. Green, E. Abramson, and R. W. Field have observed anomalous behavior of the anticrossing density in the Zeeman anticrossing (ZAC) spectra of gas phase A 1Au acetylene in the 42 200 to 45 300 cm-1 energy range. To best explain this result, they hypothesize a large singlet–triplet coupling due to the existence of a linear isomerization barrier connecting a triplet-excited cis- and trans-acetylene in the vicinity of the studied energy range (∼45 500 cm-1). Theoretically such a linear stationary point, however, must have two different degenerate bending vibrational frequencies which are either imaginary or exactly zero. Neither case has yet been experimentally detected. Here, we have studied the two lowest-lying linear triplet-excited-state stationary points of acetylene, 3Σ+u and 3Δu, to see if they fit Dupré et al.’s hypothesis. We have completed geometry optimization and harmonic vibrational frequency analysis using complete-active-space self-consistent field (CASSCF) wave functions as well as determined energy points at those geometries using the second-order configuration interaction (SOCI) method. Harmonic vibrational analyses of both stationary points reveal two different doubly degenerate vibrational modes with imaginary vibrational frequencies (or negative force constants) indicating that they are indeed saddle points with a Hessian index of four. At the DZP SOCI//CASSCF level of theory with zero-point vibrational energy (ZPVE) correction, the 3Σ+u stationary point lies 35 840 cm-1 above the ground state of acetylene. This is much too low in energy to contribute to the ZAC spectral anomaly. At the same level of theory with ZPVE correction, the 3Δu stationary point lies 44 940 cm-1 above the ground state consistent with Dupré et al.’s hypothesis. Several solutions to the anomalous ZAC spectra are discussed. We propose that the anomaly may also be due to... [ABSTRACT FROM AUTHOR]

Investigates size-consistent wave functions for nondynamical correlation energy. Definition of nondynamical correlation energy; Introduction of approximations based on coupled-cluster theory; Implementation of valence optimized orbital coupled-cluster doubles model.

Compares the molecular geometry and harmonic vibrational frequency predictions in a configuration interaction (CI) wave function for hydrogen sulfide. Inclusion of all single and double excitations and certain triple and quaruple excitations chosen in a priori manner; Harmonic vibrational frequencies and zero-point energies.

PSI3 is a program system and development platform for ab initio molecular electronic structure computations. The package includes mature programming interfaces for parsing user input, accessing commonly used data such as basis-set information or molecular orbital coefficients, and retrieving and storing binary data (with no software limitations on file sizes or file-system-sizes), especially multi-index quantities such as electron repulsion integrals. This platform is useful for the rapid implementation of both standard quantum chemical methods, as well as the development of new models. Features that have already been implemented include Hartree-Fock, multiconfigurational self-consistent-field, second-order Møller-Plesset perturbation theory, coupled cluster, and configuration interaction wave functions. Distinctive capabilities include the ability to employ Gaussian basis functions with arbitrary angular momentum levels; linear R12 second-order perturbation theory; coupled cluster frequency-dependent response properties, including dipole polarizabilities and optical rotation; and diagonal Born-Oppenheimer corrections with correlated wave functions. This article describes the programming infrastructure and main features of the package. PSI3 is available free of charge through the open-source, GNU General Public License. © 2007 Wiley Periodicals, Inc. J Comput Chem, 2007 [ABSTRACT FROM AUTHOR]

We recently reported that very compact coupled-cluster wave functions may be generated by selecting the most important configurations, by weight, from the full coupled-cluster wave function. Here, we consider how the choice of orbitals may affect these wave functions in the case of the symmetric dissociation of H 2 O. We employ unrestricted Hartree–Fock and complete-active-space self-consistent-field orbitals, as well as natural orbitals derived from a coupled-cluster singles and doubles wave function. For a given accuracy, some choices of orbitals can reduce the size of configuration interaction wave functions, but they have little effect on the weight-selected coupled-cluster wave functions. [ABSTRACT FROM AUTHOR]

Minimal reference space definitions are presented for complete active space self-consistent field (CASSCF) and restricted active space self-consistent field (RASSCF) descriptions of bond-breaking reactions and compared to CASSCF wavefunctions using full-valence and one-to-one active spaces. Potential energy curves for reactions breaking both single and multiple bonds are computed using multi-reference perturbation theory and multi-reference configuration interaction based upon these reference wavefunctions, and results are compared to full configuration interaction benchmark curves. This allows one to ascertain the ability of minimal reference spaces to recover the strong non-dynamical correlation effects in these systems by comparison to results with larger reference spaces and to the exact results for a given basis set. Minimal reference spaces, at a fraction of the cost, typically perform on par with much larger reference spaces in computations which also include dynamical electron correlation. [ABSTRACT FROM AUTHOR]

The mean-field ENMO (electronic and nuclear molecular orbitals) wave function is written as a product of one-particle functions that pertain to different types of particles, antisymmetrized over the fermionic orbitals, thus allowing the simultaneous treatment of electrons and nuclei within a self-consistent framework. We implemented the ENMO-SCF (self-consistent field) as well as correlated versions of ENMO analogous to the MBPT (many-body perturbation theory) and CI (configuration integration) models of Born-Oppenheimer quantum chemistry. The ENMO methods are attractive by virtue of their similarity to traditional electronic structure theory models, and several researchers have recently implemented such approaches. Here we present the first fundamental study of the convergence of ENMO methods with respect to basis set and correlation treatment for simple systems. It is found that the elimination of the translational invariance of the full molecular Hamiltonian is pivotal for obtaining a physically meaningful spectrum. For example, the ENMO-full CI spectrum for the hydrogen atom contains a manifold of spurious states which cannot be removed by simple subtraction of centre-of-mass kinetic energy because of the general non-factorizability (non-separability) of the ENMO ansatz into internal and centre-of-mass parts. An alternative, hybrid approach that treats only some light nuclei in a molecule quantum-mechanically avoids the spurious states problem by breaking the translational invariance of the Hamiltonian. However, both simple analytical estimation and numerical examples for some diatomic hydrides show that vibrational energy levels are not accurately predicted in this approach; this suggests difficulty in using such methods to predict tunnelling splittings. Finally, slow convergence with respect to basis set and correlation method is observed and related to the failure of ENMO methods to describe interparticle cusps (including the electron/nuclear cusp) efficiently due to the lack of explicit interparticle coordinates. We elevate these previously unrecognized or underappreciated difficulties as being of prime importance to future progress in ENMO methods. [ABSTRACT FROM AUTHOR]

The manipulation of the rank-four tensor of double excitation amplitudes represents a challenge to the efficient implementation of many electronic structure methods. We present a proof of concept for the approximation of doubles amplitudes in the tensor hypercontraction (THC) representation. In particular, we show how THC can be used to both reduce the scaling with respect to molecular size of coupled cluster singles and doubles (CCSD) (and related methods) by two orders [from O(N6) to O(N4)] and remove the memory bottleneck associated with storage of the doubles amplitudes. The accuracy of correlated methods as integral and amplitude approximations are introduced is examined. For a set of 20 small molecules, single and double-excitation configuration interaction (CISD), quadratic CISD (QCISD), and CCSD correlation energies could be reproduced with millihartree accuracy after the introduction of these approximations. Our approach exploits otherwise hidden factorizable tensor structure in both the electron repulsion integrals and the wavefunction coefficients and should be applicable with suitable modifications to many methods in electronic structure theory. [ABSTRACT FROM AUTHOR]

We present a general perturbative method for correcting a singles and doubles coupled-cluster energy. The coupled-cluster wave function is used to define a similarity-transformed Hamiltonian, which is partitioned into a zeroth-order part that the reference problem solves exactly plus a first-order perturbation. Standard perturbation theory through second-order provides the leading correction. Applied to the valence optimized doubles (VOD) approximation to the full-valence complete active space self-consistent field method, the second-order correction, which we call (2), captures dynamical correlation effects through external single, double, and semi-internal triple and quadruple substitutions. A factorization approximation reduces the cost of the quadruple substitutions to only sixth order in the size of the molecule. A series of numerical tests are presented showing that VOD(2) is stable and well-behaved provided that the VOD reference is also stable. The second-order correction is also general to standard unwindowed coupled-cluster energies such as the coupled-cluster singles and doubles (CCSD) method itself, and the equations presented here fully define the corresponding CCSD(2) energy. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]