An efficient linear-scaling CCSD(T) method based on local natural orbitals

C2 in a Box: Determining Its Intrinsic Bond Strength for the X(1)Σg(+) Ground State

The intrinsic bond strength of C2 in its (1)Σg(+) ground state is determined from its stretching force constant utilizing MR-CISD+Q(8,8), MR-AQCC(8,8), and single-determinant coupled cluster calculations with triple and quadruple excitations. By referencing the CC stretching force constant to its local counterparts of ethane, ethylene, and acetylene, an intrinsic bond strength half way between that of a double bond and a triple bond is obtained. Diabatic MR-CISD+Q results do not change this. Confinement of C2 and suitable reference molecules in a noble gas cage leads to compression, polarization, and charge transfer effects, which are quantified by the local CC stretching force constants and differences of correlated electron densities. These results are in line with two π bonds and a partial σ bond. Bond orders and bond dissociation energies of small hydrocarbons do not support quadruple bonding in C2.

Putting David Craig’s Legacy to Work in Nanotechnology and Biotechnology

David Craig (1919–2015) left us with a lasting legacy concerning basic understanding of chemical spectroscopy and bonding. This is expressed in terms of some of the recent achievements of my own research career, with a focus on integration of Craig’s theories with those of Noel Hush to solve fundamental problems in photosynthesis, molecular electronics (particularly in regard to the molecules synthesized by Maxwell Crossley), and self-assembled monolayer structure and function. Reviewed in particular is the relation of Craig’s legacy to: the 50-year struggle to assign the visible absorption spectrum of arguably the world’s most significant chromophore, chlorophyll; general theories for chemical bonding and structure extending Hush’s adiabatic theory of electron-transfer processes; inelastic electron-tunnelling spectroscopy (IETS); chemical quantum entanglement and the Penrose–Hameroff model for quantum consciousness; synthetic design strategies for NMR quantum computing; Gibbs free-energy measurements and calculations for formation and polymorphism of organic self-assembled monolayers on graphite surfaces from organic solution; and understanding the basic chemical processes involved in the formation of gold surfaces and nanoparticles protected by sulfur-bound ligands, ligands whose form is that of Au0-thiyl rather than its commonly believed AuI-thiolate tautomer.

First UHF Implementation of the Incremental Scheme for Open-Shell Systems

The incremental scheme makes it possible to compute CCSD(T) correlation energies to high accuracy for large systems. We present the first extension of this fully automated black-box approach to open-shell systems using an Unrestricted Hartree-Fock (UHF) wave function, extending the efficient domain-specific basis set approach to handle open-shell references. We test our approach on a set of organic and metal organic structures and molecular clusters and demonstrate standard deviations from canonical CCSD(T) values of only 1.35 kJ/mol using a triple ζ basis set. We find that the incremental scheme is significantly more cost-effective than the canonical implementation even for relatively small systems and that the ease of parallelization makes it possible to perform high-level calculations on large systems in a few hours on inexpensive computers. We show that the approximations that make our approach widely applicable are significantly smaller than both the basis set incompleteness error and the intrinsic error of the CCSD(T) method, and we further demonstrate that incremental energies can be reliably used in extrapolation schemes to obtain near complete basis set limit CCSD(T) reaction energies for large systems.

Construction and application of a new dual-hybrid random phase approximation

The direct random phase approximation (dRPA) combined with Kohn-Sham reference orbitals is among the most promising tools in computational chemistry and applicable in many areas of chemistry and physics. The reason for this is that it scales as N(4) with the system size, which is a considerable advantage over the accurate ab initio wave function methods like standard coupled-cluster. dRPA also yields a considerably more accurate description of thermodynamic and electronic properties than standard density-functional theory methods. It is also able to describe strong static electron correlation effects even in large systems with a small or vanishing band gap missed by common single-reference methods. However, dRPA has several flaws due to its self-correlation error. In order to obtain accurate and precise reaction energies, barriers and noncovalent intra- and intermolecular interactions, we construct a new dual-hybrid dRPA (hybridization of exact and semilocal exchange in both the energy and the orbitals) and test the performance of this new functional on isogyric, isodesmic, hypohomodesmotic, homodesmotic, and hyperhomodesmotic reaction classes. We also use a test set of 14 Diels-Alder reactions, six atomization energies (AE6), 38 hydrocarbon atomization energies, and 100 reaction barrier heights (DBH24, HT-BH38, and NHT-BH38). For noncovalent complexes, we use the NCCE31 and S22 test sets. To test the intramolecular interactions, we use a set of alkane, cysteine, phenylalanine-glycine-glycine tripeptide, and monosaccharide conformers. We also discuss the delocalization and static correlation errors. We show that a universally accurate description of chemical properties can be provided by a large, 75% exact exchange mixing both in the calculation of the reference orbitals and the final energy.

The ground-state potential energy curve of the radium dimer from relativistic coupled cluster calculations

The potential energy curve for the ground-state of radium dimer (Ra2) is provided by means of atomic and molecular relativistic coupled cluster calculations. The short-range part of this curve is defined by an equilibrium bond length of 5.324 Å, a dissociation energy of 897 cm(-1), and a harmonic vibrational frequency of 20.5 cm(-1). The asymptotic behavior at large interatomic distances is characterized by the van der Waals coefficients C6 = 5.090 × 10(3), C8 = 6.978 × 10(5), and C10 = 8.786 × 10(7) atomic units. The two regions are matched in an analytical potential to provide a convenient representation for use in further calculations, for instance, to model cold collisions between radium atoms. This might become relevant in future experiments on ultracold, optically trapped, radioactive radium atoms that are used to search for a permanent electric dipole moment.

The π-Electron Delocalization in 2-Oxazolines Revisited: Quantification and Comparison with Its Analogue in Esters

The single crystal X-ray analysis of the ester-functionalized 2-oxazoline, methyl 3-(4,5-dihydrooxazol-2-yl)propanoate, revealed π-electron delocalization along the N–C–O segment in the 2-oxazoline pentacycle to significant extent, which is comparable to its counterpart along the O–C–O segment in the ester. Quantum chemical calculations based on the experimental X-ray geometry of the molecule supported the conjecture that the N–C–O segment has a delocalized electronic structure similar to an ester group. The calculated bond orders were 1.97 and 1.10 for the N=C and C–O bonds, and the computed partial charges for the nitrogen and oxygen atoms of −0.43 and −0.44 were almost identical. In the ester group, the bond orders were 1.94 and 1.18 for the C–O bonds, while the partial charges of the oxygen atom are −0.49 and −0.41, which demonstrates the similar electronic structure of the N–C–O and O–C–O segments. In 2-oxazolines, despite the higher electronegativity of the oxygen atom (compared to the nitrogen atom), the charges of the hetero atoms oxygen and nitrogen are equalized due to the delocalization, and it also means that a cationic attack on the nitrogen is possible, enabling regioselectivity during the initiation of the cationic ring-opening polymerization of 2-oxazoline monomers, which is a prerequisite for the synthesis of materials with well-defined structures.

A systematic way for the cost reduction of density fitting methods

We present a simple approach for the reduction of the size of auxiliary basis sets used in methods exploiting the density fitting (resolution of identity) approximation for electron repulsion integrals. Starting out of the singular value decomposition of three-center two-electron integrals, new auxiliary functions are constructed as linear combinations of the original fitting functions. The new functions, which we term natural auxiliary functions (NAFs), are analogous to the natural orbitals widely used for the cost reduction of correlation methods. The use of the NAF basis enables the systematic truncation of the fitting basis, and thereby potentially the reduction of the computational expenses of the methods, though the scaling with the system size is not altered. The performance of the new approach has been tested for several quantum chemical methods. It is demonstrated that the most pronounced gain in computational efficiency can be expected for iterative models which scale quadratically with the size of the fitting basis set, such as the direct random phase approximation. The approach also has the promise of accelerating local correlation methods, for which the processing of three-center Coulomb integrals is a bottleneck.

Incremental evaluation of coupled cluster dipole polarizabilities

In this work we present the first implementation of the incremental scheme for coupled cluster linear-response frequency-dependent dipole polarizabilities.

Spin-rotation and NMR shielding constants in XF molecules (X = B, Al, Ga, In, and Tl)

Accurate spin-rotation and absolute shielding constants in a series of XF molecules (X = (11)B, (27)Al, (69)Ga, (115)In, and (205)Tl) determined using high-level ab initio coupled-cluster and four-component relativistic density-functional theory (DFT) calculations are presented. The accuracy of the results is established by comparing the relativistically and vibrationally corrected calculated values with available experimental data; for spin-rotation and shielding constants for which no experimental data exist, we provide new and reliable values. For both properties, our results can be considered as reference values against which more approximate quantum-chemical methods can be benchmarked.

Improved cluster-in-molecule local correlation approach for electron correlation calculation of large systems

An improved cluster-in-molecule (CIM) local correlation approach is developed to allow electron correlation calculations of large systems more accurate and faster. We have proposed a refined strategy of constructing virtual LMOs of various clusters, which is suitable for basis sets of various types. To recover medium-range electron correlation, which is important for quantitative descriptions of large systems, we find that a larger distance threshold (ξ) is necessary for highly accurate results. Our illustrative calculations show that the present CIM-MP2 (second-order Møller-Plesser perturbation theory, MP2) or CIM-CCSD (coupled cluster singles and doubles, CCSD) scheme with a suitable ξ value is capable of recovering more than 99.8% correlation energies for a wide range of systems at different basis sets. Furthermore, the present CIM-MP2 scheme can provide reliable relative energy differences as the conventional MP2 method for secondary structures of polypeptides.

On the protonation of water

Dissociative photoionization onsets of water and water dimer, measured by Imaging Photoelectron Photoion Coincidence (iPEPICO) Spectroscopy, are used in a floating thermochemical cycle to determine the proton affinity of water with unprecedented accuracy, as confirmed by state-of-the-artab initioquantum-chemical calculations.

Chemical applications carried out by local pair natural orbital based coupled-cluster methods

Local pair natural orbital coupled-cluster methods are suitable for accurate assessment of the energies of large molecular and supramolecular systems.