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Nonlinear optical properties of the rhombic B4 -cluster
1Institute of Organic and Pharmaceutical Chemistry, National Hellenic Research Foundation, Vasileos Constantinou 48, GR-11635 Athens, Greece.
Journal of Computational Chemistry
|August 11, 2021
Summary
Multireference methods reveal excited state contributions to the ground state wave function of the rhombic B4 cluster. Correlated methods provide reliable electric properties, though differences exist between CCSD(T) and BD(T).
Area of Science:
- Computational Chemistry
- Quantum Chemistry
- Materials Science
Background:
- Accurate calculation of electronic structure and properties for small clusters is crucial for understanding their behavior.
- The rhombic B4 cluster, with D2h symmetry, presents a computational challenge due to potential multireference character.
Purpose of the Study:
- To investigate the equilibrium geometry, energy, dipole polarizability, and second hyperpolarizability of the rhombic B4 cluster.
- To compare the performance of various quantum chemical methods, including single-reference and multireference approaches.
Main Methods:
- Employed restricted (RHF) and unrestricted (UHF) Hartree-Fock, Møller-Plesset perturbation theory (MPn), coupled-cluster theory [CCSD(T), BD(T)], and complete/restricted active space SCF (CASSCF/RASSCF).
- Utilized three distinct basis sets to assess the impact of basis set size on calculated properties.
- Investigated the influence of excited state configurations on the ground state wave function.
Main Results:
- Multireference methods indicated significant contributions from excited state configurations to the ground state wave function.
- Restricted Hartree-Fock (RHF) results deviated substantially from correlated methods, even for geometry.
- Unrestricted Hartree-Fock (UHF) provided more reliable geometries but exhibited spin contamination; correlated methods yielded comparable electric properties, with notable differences between CCSD(T) and BD(T).
Conclusions:
- The ground state of the B4 cluster possesses multireference character, necessitating the use of advanced correlated or multireference methods for accurate predictions.
- Electric properties are qualitatively similar across different correlated methods, but quantitative differences warrant careful selection of the theoretical approach.
- Basis set extension with f-functions had a minimal impact on the calculated electric properties.
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