Abstract
In this work, a detailed study of spin-state splittings in three spin crossover model compounds with DLPNO-CCSD(T) is presented. The performance in comparison to canonical CCSD(T) is assessed in detail. It was found that spin-state splittings with chemical accuracy, compared to the canonical results, are achieved when the full iterative triples (T1) scheme and TightPNO settings are applied and relativistic effects are taken into account. Having established the level of accuracy that can be reached relative to the canonical results, we have undertaken a detailed basis set study in the second part of the study. The slow convergence of the results of correlated calculations with respect to basis set extension is particularly acute for spin-state splittings for reasons discussed in detail in this Article. In fact, for some of the studied systems, 5Z basis sets are necessary in order to come close to the basis set limit that is estimated here by basis set extrapolation. Finally, the results of the present work are compared to available literature. In general, acceptable agreement with previous CCSD(T) results is found, although notable deviations stemming from differences in methodology and basis sets are noted. It is noted that the published CASPT2 numbers are far away from the extrapolated CCSD(T) numbers. In addition, dynamic quantum Monte Carlo results differ by several tens of kcal/mol from the CCSD(T) numbers. A comparison to DFT results produced with a range of popular density functionals shows the expected scattering of results and showcases the difficulty of applying DFT to spin-state energies.
Highlights
IntroductionThe relative energies of different spin states in transition metal complexes, called spin-state splittings, are of central importance for fields like spin crossover, magnetism, and bioinorganic chemistry.[1−3] Here, multistate reactivity and catalysis pose formidable challenges for established computational methods such as density functional theory (DFT) which has been evaluated in numerous studies.[4−6] On the other hand, wave function theory (WFT) methods, though systematically improvable and in principle, as accurate as one desires, often suffer from their complexity, nonblackbox character, and, most of all, the often overwhelming computational expense.[7−9]It is not at all trivial to decide which method one should apply when faced with the task of computing accurate spinstate energies of a given compound.[10,11]We would like to briefly reiterate the underlying physics of the problem at hand
Since it has previously been shown that correlation of the semicore electrons and scalar relativistic effects contribute in a nonnegligible fashion to spin-state splittings,[53] we chose to include both effects and adopt the necessary basis sets, i.e., cc-pVnZ
Our results demonstrate that these technical shortcomings have been remedied and that, with sufficient care, results within 1 kcal/mol of the canonical results can be obtained with DLPNO-CCSD(T1)
Summary
The relative energies of different spin states in transition metal complexes, called spin-state splittings, are of central importance for fields like spin crossover, magnetism, and bioinorganic chemistry.[1−3] Here, multistate reactivity and catalysis pose formidable challenges for established computational methods such as density functional theory (DFT) which has been evaluated in numerous studies.[4−6] On the other hand, wave function theory (WFT) methods, though systematically improvable and in principle, as accurate as one desires, often suffer from their complexity, nonblackbox character, and, most of all, the often overwhelming computational expense.[7−9]It is not at all trivial to decide which method one should apply when faced with the task of computing accurate spinstate energies of a given compound.[10,11]We would like to briefly reiterate the underlying physics of the problem at hand. The relative energies of different spin states in transition metal complexes, called spin-state splittings, are of central importance for fields like spin crossover, magnetism, and bioinorganic chemistry.[1−3] Here, multistate reactivity and catalysis pose formidable challenges for established computational methods such as density functional theory (DFT) which has been evaluated in numerous studies.[4−6] On the other hand, wave function theory (WFT) methods, though systematically improvable and in principle, as accurate as one desires, often suffer from their complexity, nonblackbox character, and, most of all, the often overwhelming computational expense.[7−9] It is not at all trivial to decide which method one should apply when faced with the task of computing accurate spinstate energies of a given compound.[10,11]. The latter, in the variant including single, double, and perturbative triple excitations (CCSD(T)), is often called the gold standard of quantum chemistry.[12,13] Dynamical correlation in systems with substantial multiconfigurational characters are usually
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