Relativistic Internally Contracted Multireference Electron Correlation Methods

TL;DR

This paper introduces advanced relativistic multireference electron correlation methods based on the Dirac Hamiltonian, enabling accurate simulations of molecules with heavy elements, demonstrated through spectroscopic calculations.

Contribution

It presents the development and implementation of relativistic ic-MRCI, CASPT2, and NEVPT2 methods using an automatic code generator for tensor equations, applied to heavy-element molecules.

Findings

Accurate rovibrational transition energies for HI and TlH

Effective simulation of relativistic electronic structures

Validation of methods through spectral comparisons

Abstract

We report internally contracted relativistic multireference configuration interaction (ic-MRCI), complete active space second-order perturbation (CASPT2), and strongly contracted n-electron valence state perturbation theory (NEVPT2) on the basis of the four-component Dirac Hamiltonian, enabling accurate simulations of relativistic, quasi-degenerate electronic structure of molecules containing transition-metal and heavy elements. Our derivation and implementation of ic-MRCI and CASPT2 are based on an automatic code generator that translates second-quantized ansatze to tensor-based equations, and to efficient computer code. NEVPT2 is derived and implemented manually. The rovibrational transition energies and absorption spectra of HI and TlH are presented to demonstrate the accuracy of these methods.