Linear scaling perturbative triples correction approximations for open-shell domain-based local pair natural orbital coupled cluster singles and doubles theory [DLPNO-CCSD(T/T)].

The coupled cluster method with single-, double-, and perturbative triple excitations [CCSD(T)] is considered to be one of the most reliable quantum chemistry theories. However, the steep scaling of CCSD(T) has limited its application to small or medium-sized systems for a long time. In our previous work, the linear scaling domain based local pair natural orbital CCSD variant (DLPNO-CCSD) has been developed for closed-shell and open-shell.

Comprehensive Benchmark Results for the Domain Based Local Pair Natural Orbital Coupled Cluster Method (DLPNO-CCSD(T)) for Closed- and Open-Shell Systems.

In this study we examine the accuracy of domain-based local pair natural orbital coupled cluster theory with single, double, and perturbative triple excitations (DLPNO-CCSD(T)) on a large benchmark data set. To this end, we use the recently published GMTKN55 superset of molecules that contains 1505 relative energies and 2462 single-point calculations. To our knowledge this is the most comprehensive benchmark evaluation of any highly correlated wave function based ab initio method to date.

Communication: An improved linear scaling perturbative triples correction for the domain based local pair-natural orbital based singles and doubles coupled cluster method [DLPNO-CCSD(T)].

In this communication, an improved perturbative triples correction (T) algorithm for domain based local pair-natural orbital singles and doubles coupled cluster (DLPNO-CCSD) theory is reported. In our previous implementation, the semi-canonical approximation was used and linear scaling was achieved for both the DLPNO-CCSD and (T) parts of the calculation. In this work, we refer to this previous method as DLPNO-CCSD(T) to emphasize the semi-canonical approximation.

Exploring the Accuracy Limits of Local Pair Natural Orbital Coupled-Cluster Theory.

The domain based local pair natural orbital coupled cluster method with single-, double-, and perturbative triple excitations (DLPNO–CCSD(T)) is an efficient quantum chemical method that allows for coupled cluster calculations on molecules with hundreds of atoms. Because coupled-cluster theory is the method of choice if high-accuracy is needed, DLPNO–CCSD(T) is very promising for large-scale chemical application. However, the various approximations that have to be introduced in order to reach near linear scaling also introduce limited deviations from the canonical results.

Is It Possible To Obtain Coupled Cluster Quality Energies at near Density Functional Theory Cost? Domain-Based Local Pair Natural Orbital Coupled Cluster vs Modern Density Functional Theory.

The recently developed domain-based local pair natural orbital coupled cluster theory with single, double, and perturbative triple excitations (DLPNO-CCSD(T)) delivers results that are closely approaching those of the parent canonical coupled cluster method at a small fraction of the computational cost. A recent extended benchmark study established that, depending on the three main truncation thresholds, it is possible to approach the canonical CCSD(T) results within 1 kJ (default setting, TightPNO), 1 kcal/mol (default setting, NormalPNO), and 2-3 kcal (default setting, LoosePNO). Although thresholds for calculations with TightPNO are 2-4 times slower than those based on NormalPNO thresholds, they are still many orders of magnitude faster than canonical CCSD(T) calculations, even for small and medium sized molecules where there is little locality.

Domain Based Pair Natural Orbital Coupled Cluster Studies on Linear and Folded Alkane Chains.

In this study the question of what is the last unbranched alkane that prefers a linear conformation over a folded one is revisited from a theoretical point of view. Geometries have been optimized carefully using the most accurate theoretical approach available to date for such systems, namely, doubly hybrid density functional theory in conjunction with larger quadruple-ζ quality basis sets. The resulting geometries deviate significantly from previously reported ones and have a significant impact on the predicted energetics.

On the reaction mechanism of the complete intermolecular O2 transfer between mononuclear nickel and manganese complexes with macrocyclic ligands.

The recently described intermolecular O2 transfer between the side-on Ni-O2 complex [(12-TMC)Ni-O2](+) and the manganese complex [(14-TMC)Mn](2+), where 12-TMC and 14-TMC are 12- and 14-membered macrocyclic ligands, 12-TMC=1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane and 14-TMC=1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane, is studied by means of DFT methods. B3LYP calculations including long-range corrections and solvent effects are performed to elucidate the mechanism. The potential energy surfaces (PESs) compatible with different electronic states of the reactants have been analyzed.

Excitation wavelength dependent O2 release from copper(II)-superoxide compounds: laser flash-photolysis experiments and theoretical studies.

Irradiation of the copper(II)-superoxide synthetic complexes [(TMG3tren)Cu(II)(O2)](+) (1) and [(PV-TMPA)Cu(II)(O2)](+) (2) with visible light resulted in direct photogeneration of O2 gas at low temperature (from -40 °C to -70 °C for 1 and from -125 to -135 °C for 2) in 2-methyltetrahydrofuran (MeTHF) solvent. The yield of O2 release was wavelength dependent: λexc = 436 nm, ϕ = 0.29 (for 1), ϕ = 0.

Improved correlation energy extrapolation schemes based on local pair natural orbital methods.

It is well-known that the basis set limit is difficult to reach in correlated post Hartree-Fock ab initio calculations. One possible route forward is to employ basis set extrapolation schemes. In order to avoid prohibitively expensive calculations, the highest level calculation (typically based on the "gold standard" coupled cluster theory with single, double, and perturbative triple excitations, CCSD(T)) is only performed with the smallest basis set, and the remaining basis set incompleteness is estimated at a lower level of theory, typically second-order Möller-Plesset perturbation theory (MP2).

Efficient and accurate local single reference correlation methods for high-spin open-shell molecules using pair natural orbitals.

A production level implementation of the high-spin open-shell (spin unrestricted) single reference coupled pair, quadratic configuration interaction and coupled cluster methods with up to doubly excited determinants in the framework of the local pair natural orbital (LPNO) concept is reported. This work is an extension of the closed-shell LPNO methods developed earlier [F. Neese, F.

Theoretical elucidation of a classic reaction: protonation of the quadruple bond of the octachlorodimolybdate(II,II) [Mo2Cl8]4- anion.

The protonation reaction of the unbridged quadruple metal-metal bond of [Mo(2)Cl(8)](4-) anion producing the triply bonded hydride [Mo(2)(μ-H)(μ-Cl)(2)Cl(6)](3-) is studied by accurate Density Functional Theory computations. The reactant, product, stable intermediates, and transition states are located on the potential energy surface. The water solvent is explicitly included in the calculations.

Protein-ligand interaction energies with dispersion corrected density functional theory and high-level wave function based methods.

With dispersion-corrected density functional theory (DFT-D3) intermolecular interaction energies for a diverse set of noncovalently bound protein-ligand complexes from the Protein Data Bank are calculated. The focus is on major contacts occurring between the drug molecule and the binding site. Generalized gradient approximation (GGA), meta-GGA, and hybrid functionals are used.

Interplay of Correlation and Relativistic Effects in Correlated Calculations on Transition-Metal Complexes: The (Cu2O2)(2+) Core Revisited.

Owing to the availability of large-scale computing facilities and the development of efficient new algorithms, wave function-based ab initio calculations are becoming more common in bioinorganic chemistry. In principle they offer a systematic route toward high accuracy. However, these calculations are by no means trivial.

Correlated wavefunction methods in bioinorganic chemistry.

In this commentary the challenges faced in the application of wavefunction-based ab initio methods to (open-shell) transition metal complexes of (bio)inorganic interest are briefly touched on. Both single-reference and multireference methods are covered. It is stressed that the generation and nature of the reference wavefunction is a subject of major importance.

Weak Molecular Interactions Studied with Parallel Implementations of the Local Pair Natural Orbital Coupled Pair and Coupled Cluster Methods.

A parallel implementation of the recently developed local pair natural orbital coupled electron pair approximation (LPNO-CEPA/n, n = Version 1, 2, or 3) and the corresponding LPNO coupled cluster method with single- and double excitations (LPNO-CCSD) is described. A detailed analysis alongside pseudocode is presented for the most important computational steps. The scaling with respect to the number of processors is reasonable and speedups of about 10 with 14 processors have been found in benchmark calculations (wall-clock time).

A multiconfigurational ab initio study of the zero-field splitting in the di- and trivalent hexaquo-chromium complexes.

A detailed analysis of the value of zero-field splitting for the di- and trivalent chromium hexaquo complexes is presented. The effect of the Jahn-Teller distortion was studied, for the case of the divalent complex, through the use of state-averaged CASSCF calculations, for the mapping of the potential energy surface along the e(g) normal modes. At the minima of the surface, multiconfigurational ab initio calculations (spectroscopy oriented configuration interaction, SORCI, and difference dedicated configuration interaction, DDCI) were used for the calculation of the D tensor and the analysis of the individual contributions to it.

Efficient and accurate approximations to the local coupled cluster singles doubles method using a truncated pair natural orbital basis.

A production level implementation of the closed-shell local quadratic configuration interaction and coupled cluster methods with single and double excitations (QCISD and CCSD) based on the concept of pair natural orbitals [local pair natural orbital LPNO-QCISD and LPNO-CCSD) is reported, evaluated, and discussed. This work is an extension of the earlier developed LPNO coupled-electron pair approximation (LNPO-CEPA) method [F. Neese et al.

Theoretical investigation of the stepwise hydrolysis of the [Re(3)(mu-Cl)(3)Cl(9)](3-) anion.

A detailed study of the stepwise substitution of the chloride ligands in the [Re3(mu-Cl)3Cl9](3-) (1) anion by water molecules is presented using theoretical methods. Ligand lability as well as the structure and relative stability of the various mono-[Re3(mu-Cl)3Cl8(H2O)](2-) (2a,b) and dihydro-[Re3(mu-Cl)3Cl7(H2O)2](-) (3a-f) conformers is examined. Clear preferences for the positions of the incoming water ligands are proposed based on calculated energy and vibrational data, which fully agree with the experimental results.