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Related Concept Videos

¹H NMR: Long-Range Coupling01:27

¹H NMR: Long-Range Coupling

The coupling interactions of nuclei across four or more bonds are usually weak, with J values less than 1 Hz. While these are usually not observed in spectra, the presence of multiple bonds along the coupling pathway can result in observable long-range coupling.
In alkenes, spin information is communicated via σ–π overlap, as seen in allylic (four-bond) and homoallylic (five-bond) couplings. These coupling interactions are stronger when the σ bond is parallel to the alkene π orbitals.
Cluster Sampling Method01:20

Cluster Sampling Method

Appropriate sampling methods ensure that samples are drawn without bias and accurately represent the population. Because measuring the entire population in a study is not practical, researchers use samples to represent the population of interest.
To choose a cluster sample, divide the population into clusters (groups) and then randomly select some of the clusters. All the members from these clusters are in the cluster sample. For example, if you randomly sample four departments from your...
Chemical Shift: Internal References and Solvent Effects01:17

Chemical Shift: Internal References and Solvent Effects

In an NMR sample, precise measurement of the absolute absorption frequencies of nuclei is difficult. A standard internal reference compound is added, and the frequency difference between the reference signal and sample signals is measured.
The internal reference compound generally used in NMR spectroscopy is tetramethylsilane (TMS). TMS is preferred because it is chemically inert, soluble in NMR solvents, and easily removable. Also, the highly shielded methyl protons in TMS yield an intense...
Multicompartment Models: Overview01:14

Multicompartment Models: Overview

Multicompartment models are mathematical constructs that depict how drugs are distributed and eliminated within the body. They segment the body into several compartments, symbolizing various physiological or anatomical areas connected through drug transfer processes such as absorption, metabolism, distribution, and elimination.
These models offer a more comprehensive representation of drug behavior in the body than one-compartment models. They accommodate the complexity of drug distribution,...
¹H NMR: Pople Notation01:09

¹H NMR: Pople Notation

The Pople nomenclature system classifies spin systems based on the difference between their chemical shifts. Coupled spins are denoted by capital letters with subscripts indicating the number of equivalent nuclei. When the coupled nuclei have well-separated chemical shifts, they are assigned letters that are far apart in the alphabet, such as A and X. When the difference in chemical shifts is small, coupled nuclei are named using adjacent letters of the alphabet (AB, MN, or XY).
A proton...
Hybridization of Atomic Orbitals I03:24

Hybridization of Atomic Orbitals I

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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Computation of Atmospheric Concentrations of Molecular Clusters from ab initio Thermochemistry
12:11

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Published on: April 8, 2020

A state-specific partially internally contracted multireference coupled cluster approach.

Dipayan Datta1, Liguo Kong, Marcel Nooijen

  • 1Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada. d3datta@uwaterloo.ca

The Journal of Chemical Physics
|June 14, 2011
PubMed
Summary
This summary is machine-generated.

A new computational method improves multireference coupled cluster calculations for complex molecules. This approach, using many-body residuals, enhances accuracy for electronic states and chemical reactions.

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Area of Science:

  • Quantum Chemistry
  • Computational Chemistry
  • Theoretical Chemistry

Background:

  • Accurate electronic structure calculations are crucial for understanding molecular properties and reactions.
  • Multireference coupled cluster (MRCC) methods are powerful but computationally demanding.
  • Existing MRCC approaches face challenges with convergence and accuracy for certain systems.

Purpose of the Study:

  • To develop a novel state-specific, partially internally contracted MRCC method.
  • To introduce a new set of residual equations, termed many-body residuals, for improved amplitude determination.
  • To enhance the accuracy and applicability of MRCC theory for general complete active spaces.

Main Methods:

  • A state-specific, partially internally contracted MRCC approach is presented.
  • Dynamical correlation is incorporated using internally contracted cluster operators acting on a multideterminantal reference.
  • A transformed Hamiltonian is diagonalized in the multireference configuration interaction singles space.
  • New many-body residual equations, derived from second quantized matrix elements, are proposed for solving cluster amplitudes.

Main Results:

  • The proposed many-body residuals avoid near-singularity, enabling the solution of all amplitudes.
  • Applications to N(2), CO, and C(2) show improved potential energy surfaces compared to conventional methods.
  • The new approach demonstrates enhanced accuracy for bond-stretching and electronic state calculations.

Conclusions:

  • The developed MRCC method with many-body residuals offers a significant improvement over existing techniques.
  • This approach provides a more robust and accurate way to study electron correlation in complex molecular systems.
  • Further investigation into minor convergence issues is warranted, but the method shows great promise for theoretical chemistry.