What Levels of Coupled Cluster Theory Are Appropriate for Transition Metal Systems? a Study Using near Exact Quantum Chemical Values for 3d Transition Metal Binary Compounds.

Abstract

<div>Transition metal compounds are traditionally considered to be challenging for standard quantum chemistry approximations like coupled cluster (CC) theory, which are usually employed to validate lower level methods like density functional theory (DFT). To explore this issue, we present a database of bond dissociation energies (BDEs) for 74 spin states of 69 diatomic species containing a 3d transition metal atom and a main group element, in the moderately sized def2-SVP basis. The presented BDEs appear to have an (estimated) 3σ error less than 1 kJ/mol relative to the exact solutions to the non-relativistic Born-Oppenheimer Hamiltonian. These benchmark values were used to assess the performance of a wide range of standard CC models, as the results should be beneficial for understanding the limitations of CC models for transition metal systems. We find that interactions between metals and monovalent ligands like hydride and fluoride are well described by CCSDT. Similarly, CCSDTQ appears to be adequate for bonds between metals and nominally divalent ligands like oxide and sulphide. However, interactions with polyvalent ligands like nitride and carbide are more challenging, with even CCSDTQ(P)<sub>Λ</sub> yielding errors on the scale of a few kJ/mol. We also find that many perturbative and iterative approximations to higher order terms either yield disappointing results, or actually worsen the performance relative to the baseline low level CC method, indicating that complexity does not always guarantee accuracy.</div>

The Ground State Electronic Energy of Benzene

“…Here, we will only briefly compare the methods on the basis of their common traits and differences. The adaptive sampling CI [56][57][58][59][60] (ASCI), semistochastic heat-bath CI can use the semistochastic cluster-analysis-driven FCIQMC (CAD-FCIQMC) approach, 94,95 in which, in the spirit of the externally corrected CC methods, The main results of our study are summarized in Figure 1 (with the underlying numerical data tabulated in the SI). No error bars are provided given that these are derived differently in the various methods.…”

Quantifying and understanding errors in molecular geometries

“…If we want to calculate GEO errors for larger main-group molecules, for which CCSD(T) is too expensive, we can use B2PLYP as a reference in place of CCSD(T). Multireference systems, such as transition metal dimers, are more delicate and would need a better reference than CCSD(T) [11,12] to calculate GEO.…”

Revealing the Nature of Electron Correlation in Transition Metal Complexes with Symmetry-Breaking and Chemical Intuition

“…This level of accuracy, but with respect to near-exact benchmark calculations, could only be attained via high orders of CC theory which are not scalable to larger systems. 26 Ref. 26 implies, as might be expected, that higher orders of CC theory are needed to describe increasing numbers of bonds; however, there were many exceptions to this trend.…”

“…26 Ref. 26 implies, as might be expected, that higher orders of CC theory are needed to describe increasing numbers of bonds; however, there were many exceptions to this trend. Considering even the simplest bonding motif (i.e.…”

“…Transition metal atoms with partially-filled valence d shells can exhibit substantial MR character due to the close energetic spacing of many low-lying electronic states, in contrast to the spectra of typical closed-shell organic compounds. 20 Transition metal diatomics have been studied extensively in recent years, 3,4,[20][21][22][23][24][25][26][27] and recent precise experiments from Morse et. al.…”