Four-Component Relativistic State-Specific Multireference Perturbation Theory with a Simplified Treatment of Static Correlation

Abstract

The relativistic multireference (MR) perturbative approach is one of the most successful tools for the description of computationally demanding molecular systems of heavy elements. We present here the ground state dissociation energy surfaces, equilibrium bond lengths, harmonic frequencies, and dissociation energies of Ag, Cu, Au, and I computed using the four-component (4c) relativistic spinors based state-specific MR perturbation theory (SSMRPT) with improved virtual orbital complete active space configuration interaction (IVO-CASCI) functions. The IVO-CASCI method is a simple, robust, useful and lower cost alternative to the complete active space self-consistent field approach for treating quasidegenerate situations. The redeeming features of the resulting method, termed as 4c-IVO-SSMRPT, lies in (i) manifestly size-extensivity, (ii) exemption from intruder problems, (iii) the freedom of convenient multipartitionings of the Hamiltonian, (iv) flexibility of the relaxed and unrelaxed descriptions of the reference coefficients, and (v) manageable cost/accuracy ratio. The present method delivers accurate descriptions of dissociation processes of heavy element systems. Close agreement with reference values has been found for the calculated molecular constants indicating that our 4c-IVOSSMRPT provides a robust and economic protocol for determining the structural properties for the ground state of heavy element molecules with eloquent MR character as it treats correlation and relativity on equal footing.

Single-Root Multireference Brillouin–Wigner Perturbative Approach to Excitation Energies

“…Over the decades, a large number of different variants of MRPT methods have been proposed and used in the literature, − among which the Hilbert-space state-specific MRPT (SSMRPT) method (where only one state is studied at a time) introduced more recently by Mukherjee and co-workers , and further developed by Chattopadhyay and co-workers ,,,, is growing in popularity because of its rigorous size extensivity and explicit insensitivity toward intruder states. The most unsettling issues of several extensively practiced MRPT methods are the well-known intruder-state problem (states within the framework of the perturbative protocol corresponding to excited configurations with vanishing energy denominators) and lack of size extensivity.…”

Inclusion of orbital relaxation and correlation through the unitary group adapted open shell coupled cluster theory using non-relativistic and scalar relativistic Hamiltonians to study the core ionization potential of molecules containing light to medium-heavy elements

“…We should also mention that this is not only true for core IP processes but also for more complex processes 9,[88][89][90][91][92][93] leading all the way to multi-reference theories with relativistic Hamiltonians which are being actively investigated. [94][95][96][97][98][99][100][101] We have also assessed the lowest order Quantum Electrodynamic (QED) correction to the core IP. It has been shown previously by Indelicato et al 102 and others, 103 in the context of atomic core ionizations and also valence ionizations, 104 that QED effects could be vitally important.…”