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Ab Initio Multireference Quantum Chemical Investigations of Molecules: Chemical Bonding, Spectroscopy, and Beyond

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Dr. Isuru R. Ariyarathna

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Theoretical Division, Los Alamos National Laboratory, Los Alamos, NM 87545, USA
Interests: quantum chemistry; excited state chemistry; chemical bonding; solvated electrons; chemistry of lanthanides and actinides; spectroscopy

Special Issue Information

Dear Colleagues,

Ab initio multireference methods are widely utilized in electronic structure theory. The progression of applying multireference tools for chemical discoveries is attributed to their ability of representing strongly correlated electronic structures of molecular complexes accurately. In many instances, multireference studies are performed for exploring chemical phenomena in spectroscopy, catalysis, and chemical bonding. The application of this level of theory is especially crucial for understanding chemicophysical processes associated with many transition metal-, lanthanide-, and actinide-based systems, owing to their highly correlated electronic structures. Furthermore, due to the high accuracy of multireference theoretical predictions, they are often conjoined with experimental investigations for validating experimental observations as well as tuning and guiding experiments. With today’s easily accessible high-performance computing power, the theoretical multireference investigations of chemical species have accelerated. Hence, aiming to highlight the ongoing multireference quantum chemical efforts in various fields of chemistry and physics, the current Special Issue is established.

Dr. Isuru R. Ariyarathna
Guest Editor

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Research

27 pages, 7874 KiB

Open AccessArticle

Electronic Structure of the Ground and Low-Lying States of MoLi

by Constantinos Demetriou and Demeter Tzeli

Molecules 2025, 30(13), 2874; https://doi.org/10.3390/molecules30132874 - 6 Jul 2025

Viewed by 243

Abstract

Molybdenum lithium compounds and materials are being researched and applied in cutting-edge industries; however, their bonding has not been explored in a systematic way. The present study investigates the MoLi molecule, to shed light on its bonding. Specifically, the electronic structure and bonding of the ground and 40 low-lying states of the MoLi molecule are explored, employing multireference methodologies, i.e., CASSCF and MRCISD(+Q) in conjunction with the aug-cc-pV5z(-PP) basis set. Bond distances, dissociation energies, dipole moments as well as common spectroscopic constants are given, while the potential energy curves are plotted. For the ground state, X

{}^{6}Σ_{+}

, it is found that

R_{e}

= 2.708 Å,

D_{e}

= 24.1 kcal/mol,

ω_{e}

= 316.8

{cm}^{- 1}

ω_{e} x_{e}

= 2.11

{cm}^{- 1}

, and μ = 3.63 D. Overall, the calculated states present a variety of bonds, from weak van der Waals up to the formation of 2.5 bonds. The dissociation energies of the calculated states range from 2.3 kcal/mol (

a {}^{8}{Σ^{+}}

) to 34.7 (

c {}^{4}Π

), while the bond distances range from 2.513 Å to 3.354 Å. Finally, dipole moment values up to 3.72 D are calculated. In most states, a 2s

{2 p}_{z}

hybridization on Li and a

{4 d}_{z^{2}}

{5 p}_{z}

or 5s

{5 p}_{z}

hybridization on Mo are found. Moreover, it is observed that the excited Li(

{}^{2}P

) atom forms the shortest bonds because its empty

{2 s}^{0}

orbital can easily accept electrons, resulting in a strong σ dative bond. Finally, the present work highlights the exceptional ability of lithium atoms to participate in a variety of bonding schemes, and it could provide the opening gate for further investigation of this species or associated material and complexes. Full article

(This article belongs to the Special Issue Ab Initio Multireference Quantum Chemical Investigations of Molecules: Chemical Bonding, Spectroscopy, and Beyond)

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20 pages, 5870 KiB

Open AccessArticle

An Ab Initio Electronic Structure Investigation of the Ground and Excited States of ScH⁺, YH⁺, and LaH⁺

by Isuru R. Ariyarathna

Molecules 2025, 30(11), 2435; https://doi.org/10.3390/molecules30112435 - 2 Jun 2025

Viewed by 544

Abstract

Multireference configuration interaction (MRCI), Davidson-corrected MRCI (MRCI+Q), coupled-cluster singles, doubles, and perturbative triples [CCSD(T)], and frozen-core full configuration interaction (fcFCI) calculations were carried out using large, correlation-consistent basis sets to investigate the excited states of the Sc atom and the spin–free and spin–orbit coupled potential energy profiles, energetics, spectroscopic constants, and electron populations of low-lying states of MH⁺ (M = Sc, Y, La). The core electron correlation effects, complete basis set effects, and spin–orbit coupling effects were also evaluated. The first four electronic states of all MH⁺ are 1²Δ, 1²Σ⁺, 1²Π, and 2²Σ⁺ with 1σ²1δ¹, 1σ²2σ¹, 1σ²1π¹, and 1σ²3σ¹ single-reference electron configurations, respectively. These states of MH⁺ can be represented by the M²⁺H^– ionic structure. The ground states of ScH⁺, YH⁺, and LaH⁺ are 1²Δ_3/2, 1²Σ⁺_1/2, and 1²Δ_3/2 with 55.45, 60.54, and 62.34 kcal/mol bond energies, respectively. The core electron correlation was found to be vital for gaining accurate predictions on the ground and excited state properties of MH⁺. The spin–orbit coupling effects are minor for ScH⁺ but become substantial moving to YH⁺ and LaH⁺. Overall, the results of this work are in good agreement with the limited set of experimental findings of MH⁺ available in the literature and will be of use for future investigations. Furthermore, the theoretical approaches, findings, and trends reported here are expected to aid studies of similar species. Full article

(This article belongs to the Special Issue Ab Initio Multireference Quantum Chemical Investigations of Molecules: Chemical Bonding, Spectroscopy, and Beyond)

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