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Radicals: Electronic Structure and Geometry01:07

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This lesson delves into the geometry of a radical, which is influenced by the electronic structure of the molecule. The principle is similar to that of a lone pair, where the unpaired electron influences the geometry at the radical center.
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Radicals, the highly reactive species, gain stability by undergoing three different reactions. The first reaction involves a radical-radical coupling, in which a radical combines with another radical, forming a spin‐paired molecule. The second reaction is between a radical and a spin‐paired molecule, generating a new radical and a new spin‐paired molecule. The third reaction is radical decomposition in a unimolecular reaction, forming a new radical and a spin‐paired...
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The mathematical expression known as the wave function, ψ, contains information about each orbital and the wavelike properties of electrons in an isolated atom. When atoms are bound together in a molecule, the wave functions combine to produce new mathematical descriptions that have different shapes. This process of combining the wave functions for atomic orbitals is called hybridization and is mathematically accomplished by the linear combination of atomic orbitals. The new orbitals that...
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An atomic orbital represents the three-dimensional regions in an atom where an electron has the highest probability to reside. The radial distribution function indicates the total probability of finding an electron within the thin shell at a distance r from the nucleus. The atomic orbitals have distinct shapes which are determined by l, the angular momentum quantum number. The orbitals are often drawn with a boundary surface, enclosing densest regions of the cloud.
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Intermediate Hamiltonian Fock-space coupled-cluster theory for excitation energies, double ionization potentials, and double electron attachments with spin-orbit coupling.

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Diffusion quantum Monte Carlo method on diradicals using single- and multi-determinant-Jastrow trial wavefunctions

Lu Rao1, Fan Wang1

  • 1Institute of Atomic and Molecular Physics, Key Laboratory of High Energy Density Physics and Technology, Ministry of Education, Sichuan University, Chengdu 610065, People's Republic of China.

The Journal of Chemical Physics
|April 2, 2022
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Diffusion Quantum Monte Carlo (DMC) accurately calculates singlet-triplet gaps for diradicals. Multi-determinant-Jastrow wavefunctions improve energy calculations, with DFT orbitals being practical for trial wavefunctions.

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Area of Science:

  • Quantum Chemistry
  • Computational Physics
  • Theoretical Chemistry

Background:

  • Diradicals are molecules with two unpaired electrons, crucial in various chemical processes.
  • Accurate calculation of their electronic states, particularly singlet and triplet states, is challenging.
  • Understanding singlet-triplet gaps is key to predicting reactivity and properties.

Purpose of the Study:

  • To employ the diffusion quantum Monte Carlo (DMC) method for calculating energies of singlet and triplet states in organic and diatomic diradicals.
  • To investigate the impact of different trial wavefunctions (SDJ, 2DJ, MDJ) and orbitals (ROB3LYP, CASSCF) on DMC energy calculations.
  • To assess the accuracy of DMC for determining singlet-triplet gaps.

Main Methods:

  • Diffusion Quantum Monte Carlo (DMC) calculations.
  • Utilized single-determinant-Jastrow (SDJ), two-determinant-Jastrow (2DJ), and multi-determinant-Jastrow (MDJ) trial wavefunctions.
  • Employed restricted open-shell B3LYP (ROB3LYP) and complete-active-space self-consistent field (CASSCF) orbitals.

Main Results:

  • DMC energies with ROB3LYP orbitals were comparable or lower for organic diradicals, less sensitive for diatomic diradicals.
  • Multi-determinant-Jastrow (MDJ) wavefunctions reduced DMC energies for most investigated diradicals.
  • Singlet-triplet gaps were calculated with a mean absolute error under 2 kcal/mol using MDJ.

Conclusions:

  • DMC, particularly with MDJ trial wavefunctions, provides accurate singlet-triplet gaps for diradicals.
  • Trial wavefunctions constructed using CASCI with DFT orbitals are practical for DMC calculations.
  • The choice of orbitals impacts DMC energies for organic diradicals more than diatomic ones.