Pair natural orbital and canonical coupled cluster reaction enthalpies involving light to heavy alkali and alkaline earth metals: the importance of sub-valence correlation

Abstract

In this work, we tested canonical and domain based pair natural orbital coupled cluster methods (CCSD(T) and DLPNO-CCSD(T), respectively) for a set of 32 ligand exchange and association/dissociation reaction enthalpies involving ionic complexes of Li, Be, Na, Mg, Ca, Sr, Ba and Pb(ii). Two strategies were investigated: in the former, only valence electrons were included in the correlation treatment, giving rise to the computationally very efficient FC (frozen core) approach; in the latter, all non-ECP electrons were included in the correlation treatment, giving rise to the AE (all electron) approach. Apart from reactions involving Li and Be, the FC approach resulted in non-homogeneous performance. The FC approach leads to very small errors (<2 kcal mol) for some reactions of Na, Mg, Ca, Sr, Ba and Pb, while for a few reactions of Ca and Ba deviations up to 40 kcal mol have been obtained. Large errors are both due to artificial mixing of the core (sub-valence) orbitals of metals and the valence orbitals of oxygen and halogens in the molecular orbitals treated as core, and due to neglecting core-core and core-valence correlation effects. These large errors are reduced to a few kcal mol if the AE approach is used or the sub-valence orbitals of metals are included in the correlation treatment. On the technical side, the CCSD(T) and DLPNO-CCSD(T) results differ by a fraction of kcal mol, indicating the latter method as the perfect choice when the CPU efficiency is essential. For completely black-box applications, as requested in catalysis or thermochemical calculations, we recommend the DLPNO-CCSD(T) method with all electrons that are not covered by effective core potentials included in the correlation treatment and correlation-consistent polarized core valence basis sets of cc-pwCVQZ(-PP) quality.

Superhalogen and Superacid

“…Most of the structures accounted are closed‐shell singlets with HOMO–LUMO gap sufficiently large to use NormalPNO cutoffs in ORCA. However, neutral X is a doublet with quite small HOMO–LUMO gap; hence, we used UHF wavefunction with TightPNO cutoffs for electron affinity determination . That choice is confirmed with the results of test computations collected in Table .…”

On the Accuracy of Density Functional Theory in Zeolite Catalysis

“…[93,94] For this cluster size, one can furthermore use canonical, 2 nd -order Møller-Plesset perturbation theory (MP2) with the def2-TZVPP basis set. Additionally, non-canonical coupled cluster methods such as the DLPNO-CCSD(T) approach [95][96][97][98] can be used employing the def2-TZVPP basis set as implemented in the Orca program.…”

“…Single points were also performed with the hybrid functionals PBE0-D3 [108] , B3LYP-D3 [126][127] and M06 [109] and Møller-Plesset second order perturbation theory MP2 [128][129][130] in Turbomole all with the def2-TZVPP/def2-QZVPP basis sets and a corresponding auxiliary basis set [131][132][133] . For CCSD(T) [129][130] both Turbomole and Orca program packages were used, Turbomole for canonical CCSD(T) calculations and Orca [93][94] to compare the accuracy of the DLPNO approximation [97][98] with the canonical values obtained for the smallest cluster model T1. Then, DLPNO CCSD(T) was carried out for the largest cluster and canonical CCSD for T1 with cc-pVXZ [134] (X=D, T, Q) basis-sets, for complete basis-set limit extrapolation (CBS) [98,[101][102]135] (details on the equations used available in the SI).…”

Gaussian basis sets for use in correlated molecular calculations. XI. Pseudopotential-based and all-electron relativistic basis sets for alkali metal (K–Fr) and alkaline earth (Ca–Ra) elements

“…In 1986, Bauschlicher et al noted that the (m1)sp outer core electrons should be correlated to obtain useful results for the alkali and alkaline-earth monohydroxides, 69 with a similar finding noted more recently by Neese and co-workers. 70 Iron et al also observed that neglecting to correlate the 3s3p for small K and Ca compounds leads "to erratic results at best." 71 This effect goes beyond that expected simply due to core-core and core-valence correlation, as in a number of cases, molecular orbitals formed as a linear combination of outer-core orbitals from the metal with valence orbitals from the heteroatom are treated as (frozen-) core orbitals in the post-HF calculation.…”