New generation of effective core potentials from correlated calculations: 2nd row elements

TL;DR

This paper develops new correlation consistent effective core potentials (ccECPs) for second row elements, achieving high accuracy in atomic excitations and molecular bonds, with options for Ne-core and He-core ECPs.

Contribution

The paper introduces optimized ccECPs for second row elements, improving transferability and accuracy in atomic and molecular calculations compared to previous ECPs.

Findings

Accurate low-lying atomic excitations within 0.03 eV

Equilibrium molecular bonds within 3 mÅ

He-core ECPs with discrepancies around 0.01 eV

Abstract

Very recently, we have introduced correlation consistent effective core potentials (ccECPs) derived from many-body approaches with the main target being its use in explicitly correlated methods but also in mainstream approaches. The ccECPs are based on reproducing excitation energies for a subset of valence states, i.e., achieving a near-isospectrality between the original and pseudo Hamiltonians. In addition, binding curves of dimer molecules have been used for refinement and overall improvement of transferability over a range of bond lengths. Here we apply similar ideas to the second row elements and study several aspects of the constructions in order to find the optimal (or nearly-optimal) solutions within the chosen ECP forms with $3s,3p$ valence space (Ne-core). New constructions exhibit accurate low-lying atomic excitations and equilibrium molecular bonds (on average within $\approx$ $0.03$ eV and $3$ m\AA), however, the errors for Al and Si oxide molecules at short bond lengths are notably larger for both ours and existing ECPs. Assuming this limitation, our ccECPs show a systematic balance between the criteria of atomic spectra accuracy and transferability for molecular bonds. In order to provide another option with much higher uniform accuracy, we also construct He-core ECPs for the whole row with typical discrepancies of $\approx$ 0.01 eV or smaller.