# Multiconfigurational Study on the Contribution of the Nondynamical and Dynamical Correlation Energies to the Dissociation Energies of Li2-to-F2 Molecules

## TL;DR

This paper investigates how different types of electron correlations affect the dissociation energies of diatomic molecules from Li2 to F2 using advanced quantum chemistry methods.

## Contribution

The study introduces a detailed multiconfigurational approach combining nondynamical and dynamical correlations for accurate dissociation energy calculations.

## Key findings

- The inclusion of nondynamical and dynamical correlations significantly improves dissociation energy accuracy for first-row diatomic molecules.
- Multiconfigurational methods like NCMET(ND) and SOCI outperform traditional approaches in capturing complex electronic interactions.
- Core correlation effects and higher excitations are essential for achieving high accuracy in dissociation energy predictions.

## Abstract

Approaching the exact solution of nonrelativistic electronic
wave
equation in molecules and the calculation of thermochemical quantities
with high accuracy, without the help of extrapolation techniques or
complicating r12 terms, is still a challenging
task in quantum chemistry. Recent advances in computer hardware made
it possible to achieve very high accuracy. However, it is still difficult
to describe many chemically important situations where the key problem
is the inherent multideterminental nature of the wave function. The
inclusion of nondynamical correlation in terms of internal correlation,
semi-internal correlation, and orbital polarization effects is crucial
in multireference systems. The multireference character of the system
at large separations, including the dissociation region, is known
to be the major difficulty for the state-of-the-art theoretical calculations.
Accurate estimation of PECs is contingent upon the proper treatment
of the intricate interplay of nondynamical and dynamical correlation
over the dissociation path. Despite their simple bonding schemes,
the homonuclear diatomic molecules of the first row atoms are still
notoriously difficult to describe from first-principles due to their
varied electronic structures. In this work, the contribution of nondynamical
and dynamical correlations to dissociation energies of diatomic molecules
of first row atoms (Li2 to F2) was investigated
in great detail. Starting from a large active space, the multiconfigurational
type NCMET(ND) wave function was calculated which includes manually
chosen nondynamical correlation terms. The def2-QZVP and correlation-consistent
quadruple-ζ basis sets (aug-cc-pVQZ) have been opted in all
calculations. A complete basis set (CBS) limit has also been calculated
through 5Z and 6Z basis sets. Internal and semi-internal type triple,
quadruple, and higher excitations were also included. The contributions
of scalar relativistic effects along with the spin–orbit interaction
to dissociation energy were taken into account. In addition, CASCI,
CASVB, SOCI, Mk-MRCC, and NCMET (nonclosed shell many-electron theory)
type calculations were performed to cover the effects of dynamical
correlation. The core correlation effects including the core polarization
phenomena and the corrections beyond adiabatic approximation were
also considered. The results were also compared to full CI computations
which are also performed in this work. The expectation values of various
one-electron properties were also tested for NCMET(ND) and SOCI methods.