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Van der Waals Interactions01:24

Van der Waals Interactions

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Atoms and molecules interact with each other through intermolecular forces. These electrostatic forces arise from attractive or repulsive interactions between particles with permanent, partial, or temporary charges. The intermolecular forces between neutral atoms and molecules are ion–dipole, dipole–dipole, and dispersion forces, collectively known as van der Waals forces.
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Van der Waals Equation01:10

Van der Waals Equation

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The ideal gas law is an approximation that works well at high temperatures and low pressures. The van der Waals equation of state (named after the Dutch physicist Johannes van der Waals, 1837−1923) improves it by considering two factors.
First, the attractive forces between molecules, which are stronger at higher densities and reduce the pressure, are considered by adding to the pressure a term equal to the square of the molar density multiplied by a positive coefficient a. Second, the volume...
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The Energies of Atomic Orbitals03:21

The Energies of Atomic Orbitals

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In an atom, the negatively charged electrons are attracted to the positively charged nucleus. In a multielectron atom, electron-electron repulsions are also observed. The attractive and repulsive forces are dependent on the distance between the particles, as well as the sign and magnitude of the charges on the individual particles. When the charges on the particles are opposite, they attract each other. If both particles have the same charge, they repel each other.
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Crystal Field Theory - Tetrahedral and Square Planar Complexes02:46

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Tetrahedral Complexes
Crystal field theory (CFT) is applicable to molecules in geometries other than octahedral. In octahedral complexes, the lobes of the dx2−y2 and dz2 orbitals point directly at the ligands. For tetrahedral complexes, the d orbitals remain in place, but with only four ligands located between the axes. None of the orbitals points directly at the tetrahedral ligands. However, the dx2−y2 and dz2 orbitals (along the Cartesian axes) overlap with the ligands less than the dxy,...
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The Van der Waals Equation01:26

The Van der Waals Equation

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The ideal gas law is based on two simplifying assumptions: first, that there are no intermolecular attractions between gas molecules, and second, that the volume occupied by the molecules themselves is negligible compared with the volume of the container. However, these assumptions don't hold up under all conditions - specifically, at high pressures and low temperatures, as gas tends to deviate from ideal gas behavior.The van der Waals equation is an enhanced version of the ideal gas law,...
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Hybridization of Atomic Orbitals II03:35

Hybridization of Atomic Orbitals II

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sp3d and sp3d 2 Hybridization
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Computation of Atmospheric Concentrations of Molecular Clusters from ab initio Thermochemistry
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Improved Complete Active Space Configuration Interaction Energies with a Simple Correction from Density Functional

Shiela Pijeau1, Edward G Hohenstein1,2

  • 1Department of Chemistry and Biochemistry, The City College of New York , New York, New York 10031, United States.

Journal of Chemical Theory and Computation
|February 4, 2017
PubMed
Summary

Density functional theory (DFT) corrects errors in complete active space configuration interaction (CASCI) calculations. This improved DFT-corrected CASCI method accurately simulates ultrafast excited-state proton transfer dynamics in large molecular systems.

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Area of Science:

  • Quantum chemistry
  • Computational chemistry
  • Theoretical chemistry

Background:

  • Complete active space configuration interaction (CASCI) methods are computationally intensive but essential for simulating molecular systems.
  • CASCI methods suffer from a neglect of dynamic electron correlation, leading to systematic overestimations of energies and barriers.
  • Accurate simulation of photochemical dynamics requires addressing these limitations.

Purpose of the Study:

  • To develop and validate a density functional theory (DFT) correction for CASCI energies.
  • To improve the accuracy of CASCI for systems with biradicaloid electronic structures and conical intersections.
  • To accurately simulate ultrafast excited-state proton transfer dynamics in large molecular systems.

Main Methods:

  • Developed a DFT correction to incorporate dynamic electron correlation into the CASCI Hamiltonian.
  • Applied the DFT-corrected CASCI approach to systems where standard DFT methods fail.
  • Investigated ultrafast excited-state proton transfer dynamics in model and large molecular systems.

Main Results:

  • The DFT correction successfully addresses the neglect of dynamic electron correlation in CASCI.
  • The DFT-corrected CASCI method accurately describes biradicaloid systems and conical intersections.
  • Simulations of excited-state proton transfer dynamics show qualitative and quantitative agreement with experimental and theoretical data.

Conclusions:

  • The DFT-corrected CASCI method overcomes limitations of standard CASCI and DFT.
  • This approach enables accurate simulations of complex photochemical processes like excited-state proton transfer.
  • The method is applicable to large molecular systems, advancing the study of condensed-phase photochemistry.