Prediction of Reaction Barriers and Thermochemical Properties with Explicitly Correlated Coupled-Cluster Methods: A Basis Set Assessment

We assessed the performance of our perturbative explicitly correlated coupled-cluster method, CCSD­(T)_F12, for accurate prediction of chemical reactivity. The reference data included reaction barrier heights, electronic reaction energies, atomization energies, and enthalpies of formation from the following sources: (1) the DBH24/08 database of 22 reaction barriers (Truhlar et al.), (2) the HJO12 set of isogyric reaction energies (Helgaker et al.), and (3) a HEAT set of atomization energies and heats of formation (Stanton et al.). We performed two types of analyses targeting the two distinct uses of explicitly correlated CCSD­(T) models: as a replacement for basis-set-extrapolated CCSD­(T) in highly accurate composite methods like HEAT and as a distinct model chemistry for standalone applications. Hence, we analyzed in detail (1) the <i>basis set error</i> of each component of the CCSD­(T)_F12 contribution to the chemical energy difference in question and (2) the <i>total error</i> of the CCSD­(T)_F12 model chemistry relative to the benchmark values. Two basis set families were utilized in the calculations: the standard aug-cc-p­(C)­VXZ-F12 (X = D, T, Q) basis sets for the conventional correlation methods and the cc-p­(C)­VXZ-F12 (X = D, T, Q) basis sets of Peterson and co-workers that are specifically designed for explicitly correlated methods. Our conclusion is that the performance of the two families for CCSD correlation contributions (which are the only components affected by the explicitly correlated terms in our formation) are nearly identical with triple- and quadruple-ζ quality basis sets, with some differences at the double-ζ level. Chemical accuracy (∼4.18 kJ/mol) for reaction barrier heights, electronic reaction energies, atomization energies, and enthalpies of formation is attained on average with the aug-cc-pVDZ, aug-cc-pVTZ, cc-pCVTZ-F12/aug-cc-pCVTZ, and cc-pCVDZ-F12 basis sets, respectively, at the CCSD­(T)_F12 level of theory. The corresponding mean unsigned errors are 1.72 kJ/mol, 1.5 kJ/mol, ∼2 kJ/mol, and 2.17 kJ/mol, and the corresponding maximum unsigned errors are 4.44 kJ/mol, 3.6 kJ/mol, ∼5 kJ/mol, and 5.75 kJ/mol

Anatomy of molecular properties evaluated with explicitly correlated electronic wave functions

<p>Static electric dipole and quadrupole moments were evaluated at the explicitly correlated second-order Møller–Plesset (MP2-F12) level for BH, CO, H₂O, and HF molecules. The electron correlation contributions to the multipole moments were further decomposed into the direct (<i>unrelaxed</i>) and indirect (<i>orbital response</i>) components; we found that both components are equally important for the conventional (MP2) contribution, whereas the F12 correction to these properties originates primarily from the orbital response effects. Finally, the direct contribution dominates in the perturbative Hartree–Fock basis set incompleteness (CABS singles) correction. Two basis set families were employed: the standard aug-cc-pVXZ series and the cc-pVXZ-F12 series designed specifically for the F12 methods. The aug-cc-pVXZ MP2-F12 multipole moments usually have smaller basis set errors than the cc-pVXZ-F12 counterparts, albeit their differences are small at the triple (<i>X</i> = <i>T</i>) and quadruple (<i>X</i> = <i>Q</i>) zeta level. With the MP2-F12 calculations, the basis set errors of dipole and quadrupole moments can be reduced to ∼0.001 a.u., or roughly 0.1%, at the aug-cc-pVDZ and aug-cc-pVTZ levels, respectively.</p

Etude semi-exclusive de la reaction &quot;4&quot;0Ar + &quot;1&quot;9&quot;7Au a 35 MeV/u : comparaison avec un modele participant-spectateur

SIGLECNRS T 61228 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

(1,2-Diaminoethane-1,2-diyl)bis(<i>N</i>‑methylpyridinium) Salts as a Prospective Platform for Designing Recyclable Prolinamide-Based Organocatalysts

A new efficient and highly recyclable organocatalyst has been developed for asymmetric cross-aldol reactions under neat conditions in ketone–ketone, ketone–aldehyde, and aldehyde–aldehyde systems. The catalyst features two prolinamide fragments and a <i>C</i>₂-symmetrical (1,2-diaminoethane-1,2-diyl)­bis­(<i>N</i>-methylpyridinium) group. The catalyst retained high activity and excellent stereoselection over the operating period of more than 830 h (25 cycles). An ab initio theoretical investigation explained the absolute configuration of the products and different stereoinduction levels for designed diastereomeric organocatalysts

Massively Parallel Implementation of Explicitly Correlated Coupled-Cluster Singles and Doubles Using TiledArray Framework

A new distributed-memory massively parallel implementation of standard and explicitly correlated (F12) coupled-cluster singles and doubles (CCSD) with canonical <i>O</i>(<i>N</i>⁶) computational complexity is described. The implementation is based on the TiledArray tensor framework. Novel features of the implementation include (a) all data greater than <i>O</i>(<i>N</i>) is distributed in memory and (b) the mixed use of density fitting and integral-driven formulations that optionally allows to avoid storage of tensors with three and four unoccupied indices. Excellent strong scaling is demonstrated on a multicore shared-memory computer, a commodity distributed-memory computer, and a national-scale supercomputer. The performance on a shared-memory computer is competitive with the popular CCSD implementations in ORCA and Psi4. Moreover, the CCSD performance on a commodity-size cluster significantly improves on the state-of-the-art package NWChem. The large-scale parallel explicitly correlated coupled-cluster implementation makes routine accurate estimation of the coupled-cluster basis set limit for molecules with 20 or more atoms. Thus, it can provide valuable benchmarks for the merging reduced-scaling coupled-cluster approaches. The new implementation allowed us to revisit the basis set limit for the CCSD contribution to the binding energy of π-stacked uracil dimer, a challenging paradigm of π-stacking interactions from the S66 benchmark database. The revised value for the CCSD correlation binding energy obtained with the help of quadruple-ζ CCSD computations, −8.30 ± 0.02 kcal/mol, is significantly different from the S66 reference value, −8.50 kcal/mol, as well as other CBS limit estimates in the recent literature

Assessment of Perturbative Explicitly Correlated Methods for Prototypes of Multiconfiguration Electronic Structure

The performance of the [2]_S and [2]_R12 universal perturbative corrections that account for one- and many-body basis set errors of single- and multiconfiguration electronic structure methods is assessed. A new formulation of the [2]_R12 methods is used in which only strongly occupied orbitals are correlated, making the approach more amenable for larger computations. Three model problems are considered using the aug-cc-pVXZ (X = D,T,Q) basis sets: the electron affinity of fluorine atom, a conformational analysis of two Si₂H₄ structures, and a description of the potential energy surfaces of the X ¹Σ_g⁺, a ³Π_u, b ³Σ_g^‑, and A ¹Π_u states of C₂. In general, the [2]_R12 and [2]_S corrections enhance energy convergence for conventional multireference configuration interaction (MRCI) and multireference perturbation theory (MRMP2) calculations compared to their complete basis set limits. For the electron affinity of the F atom, [2]_R12 electron affinities are within 0.001 eV of the experimental value. The [2]_R12 conformer relative energy error for Si₂H₄ is less than 0.1 kcal/mol compared to the complete basis set limit. The C₂ potential energy surfaces show nonparallelity errors that are within 0.7 kcal/mol compared to the complete basis set limit. The perturbative nature of the [2]_R12 and [2]_S methods facilitates the development of a straightforward text-based data exchange standard that connects an electronic structure code that can produce a two-particle density matrix with a code that computes the corrections. This data exchange standard was used to implement the interface between the GAMESS MRCI and MRMP2 codes and the MPQC [2]_R12 and [2]_S capabilities

Dirac–Coulomb–Breit Molecular Mean-Field Exact-Two-Component Relativistic Equation-of-Motion Coupled-Cluster Theory

We present a relativistic equation-of-motion coupled-cluster with single and double excitation formalism within the exact two-component framework (X2C-EOM-CCSD), where both scalar relativistic effects and spin–orbit coupling are variationally included at the reference level. Three different molecular mean-field treatments of relativistic corrections, including the one-electron, Dirac–Coulomb, and Dirac–Coulomb–Breit Hamiltonian, are considered in this work. Benchmark calculations include atomic excitations and fine-structure splittings arising from spin–orbit coupling. Comparison with experimental values and relativistic time-dependent density functional theory is also carried out. The computation of the oscillator strength using the relativistic X2C-EOM-CCSD approach allows for studies of spin–orbit-driven processes, such as the spontaneous phosphorescence lifetime