SAP-X2C: Optimally-Simple Two-Component Relativistic Hamiltonian With Size-Intensive Picture Change

TL;DR

The paper introduces SAP-X2C, a simple yet accurate relativistic 2-component Hamiltonian that models two-electron effects efficiently, suitable for large systems, and comparable in accuracy to more complex methods.

Contribution

It presents a novel SAP-X2C Hamiltonian that improves accuracy and applicability over existing 1-electron X2C methods while maintaining simplicity and computational efficiency.

Findings

SAP-X2C accurately approximates 4-component Dirac-Hartree-Fock results.

It is suitable for extended systems like large molecules and crystals.

SAP-X2C shows comparable performance to atomic mean-field X2C methods.

Abstract

We present a simple relativistic exact 2-component (X2C) Hamiltonian that models two-electron picture-change effects using Lehtola's superposition of atomic potentials (SAP) [S. Lehtola, J. Chem. Theory Comput. 15, 1593-1604 (2019)]. The SAP-X2C approach keeps the low-cost and technical simplicity of the popular 1-electron X2C (1eX2C) predecessor, but is significantly more accurate and has a well-defined thermodynamic limit, making it applicable to extended systems (such as large molecules and periodic crystals). The assessment of the SAP-X2C-based Hartree-Fock total and spinor energies, spin-orbit splittings, equilibrium bond distances, and harmonic vibrational frequencies suggests that SAP-X2C is similar to the more complex atomic mean-field (AMF) X2C counterparts in its ability to approximate the 4-component Dirac-Hartree-Fock reference.