Fock-Space Coupled Cluster Theory: Systematic Study of Partial Fourth Order Triples Schemes for Ionization Potential and Comparison with Bondonic Formalism.

Suhita Basumallick,Mihai V Putz,Sourav Pal

doi:10.3390/ijms21176199

Abstract

In this paper, we have made a systematic study of partial fourth order perturbative schemes due to triples to compute the ionization potential within Fock-space multi-reference coupled-cluster theory. In particular, we have obtained computationally less expensive correlation schemes due to fourth order triples. Prototype examples have been considered to explore the efficacy of the approximate methods mentioned, while the bondonic formalism supporting the bonding phenomenology is also respectively for the first time here advanced.

Highlights

IntroductionPhoto-ionization of electrons is a very important step to transfer the energy of radiation to matter and thus plays a crucial role in physics and chemistry [1]. Ionization of main peaks is associated with ejection of electrons from the molecular orbitals and simple Koopman’s approximation is often inadequate to describe the process of ionization. While the satellite ionization peaks cannot be explained without electron correlation, as pointed out by Cederbaum and co-workers [2], the role of electron correlation and thus the correlated theories in the calculations of main peaks has been noted in several works [3]. For the main peaks, such correlated theories improve the main peaks significantly and very often without such correlation, even the qualitative ordering of ionization peaks does not come out correctly. Several correlated theories, configuration interaction, perturbation-based, Green’s function, equation-of-motion based theories have been used for vertical ionization potential (VIP) calculation. The ones, which can produce direct calculation of VIPs by cancellation of the common ground state energies, have been preferred [4,5]. Several quantum mechanical theories are able to describe these energies as well as microscopic interactions of the systems. At the same time, theories based on bondons provide the link to the extended systems [6,7]. The bondons describe chemical particles, which are associated with electrons implicated in the chemical bond as single, delocalized, or lone pair; they have orientated direction of movement and depend on a chemical field. The bondons can be found on the wave functions, on the covalent bonds, in dispersive–weak interactions, on the mechanism of action between ligand and receptor, and even on the ionic interactions
We present in this paper new partial fourth order schemes to FS-CCSD, which is called MRCCSD, which may be useful for ionization potential (IP) calculation
We find that in the basis sets used are a [5s4p2d1f] (basis-A), the results for both 3σg and 1πu decrease with triples at third order and oscillate

Summary

Introduction

Photo-ionization of electrons is a very important step to transfer the energy of radiation to matter and thus plays a crucial role in physics and chemistry [1]. Ionization of main peaks is associated with ejection of electrons from the molecular orbitals and simple Koopman’s approximation is often inadequate to describe the process of ionization. While the satellite ionization peaks cannot be explained without electron correlation, as pointed out by Cederbaum and co-workers [2], the role of electron correlation and thus the correlated theories in the calculations of main peaks has been noted in several works [3]. For the main peaks, such correlated theories improve the main peaks significantly and very often without such correlation, even the qualitative ordering of ionization peaks does not come out correctly. Several correlated theories, configuration interaction, perturbation-based, Green’s function, equation-of-motion based theories have been used for vertical ionization potential (VIP) calculation. The ones, which can produce direct calculation of VIPs by cancellation of the common ground state energies, have been preferred [4,5]. Several quantum mechanical theories are able to describe these energies as well as microscopic interactions of the systems. At the same time, theories based on bondons provide the link to the extended systems [6,7]. The bondons describe chemical particles, which are associated with electrons implicated in the chemical bond as single, delocalized, or lone pair; they have orientated direction of movement and depend on a chemical field. The bondons can be found on the wave functions, on the covalent bonds, in dispersive–weak interactions, on the mechanism of action between ligand and receptor, and even on the ionic interactions.

Objectives

Methods

Results

Conclusion