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Related Concept Videos

Diffusion01:12

Diffusion

218.6K
Diffusion is the passive movement of substances down their concentration gradients—requiring no expenditure of cellular energy. Substances, such as molecules or ions, diffuse from an area of high concentration to an area of low concentration in the cytosol or across membranes. Eventually, the concentration will even out, with the substance moving randomly but causing no net change in concentration. Such a state is called dynamic equilibrium, which is essential for maintaining overall...
218.6K
Diffusion01:21

Diffusion

6.4K
Diffusion is a type of passive transport. In passive transport, a substance tends to move from an area of high concentration to an area of low concentration until the concentration is equal across the space. For example, take the diffusion of substances through the air. When someone opens a perfume bottle in a room filled with people, the perfume is at its highest concentration in the bottle and is at its lowest at the edges of the room. The perfume vapor will diffuse, or spread away, from the...
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Calculating the Equilibrium Constant02:46

Calculating the Equilibrium Constant

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The equilibrium constant for a reaction is calculated from the equilibrium concentrations (or pressures) of its reactants and products. If these concentrations are known, the calculation simply involves their substitution into the Kc expression.
For example, gaseous nitrogen dioxide forms dinitrogen tetroxide according to this equation:
38.0K
Calculating pH Changes in a Buffer Solution02:45

Calculating pH Changes in a Buffer Solution

58.6K
A buffer can prevent a sudden drop or increase in the pH of a solution after the addition of a strong acid or base up to its buffering capacity; however, such addition of a strong acid or base does result in the slight pH change of the solution. The small pH change can be calculated by determining the resulting change in the concentration of buffer components, i.e., a weak acid and its conjugate base or vice versa. The concentrations obtained using these stoichiometric calculations can be used...
58.6K
Calculating Standard Free Energy Changes02:49

Calculating Standard Free Energy Changes

24.9K
The free energy change for a reaction that occurs under the standard conditions of 1 bar pressure and at 298 K is called the standard free energy change. Since free energy is a state function, its value depends only on the conditions of the initial and final states of the system. A convenient and common approach to the calculation of free energy changes for physical and chemical reactions is by use of widely available compilations of standard state thermodynamic data. One method involves the...
24.9K
Calculating Equilibrium Concentrations02:05

Calculating Equilibrium Concentrations

53.4K
Being able to calculate equilibrium concentrations is essential to many areas of science and technology—for example, in the formulation and dosing of pharmaceutical products. After a drug is ingested or injected, it is typically involved in several chemical equilibria that affect its ultimate concentration in the body system of interest. Knowledge of the quantitative aspects of these equilibria is required to compute a dosage amount that will solicit the desired therapeutic effect.
A more...
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The Diffusion of Passive Tracers in Laminar Shear Flow
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Excited-State Diffusion Monte Carlo Calculations: A Simple and Efficient Two-Determinant Ansatz.

Nick S Blunt1, Eric Neuscamman2,3

  • 1University Chemical Laboratory, Lensfield Road , Cambridge CB2 1EW , United Kingdom.

Journal of Chemical Theory and Computation
|December 12, 2018
PubMed
Summary

We developed an efficient quantum chemistry method for excited states using variational Monte Carlo. This approach accurately models electronic structures, crucial for understanding molecular behavior.

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Area of Science:

  • Quantum Chemistry
  • Computational Physics
  • Theoretical Chemistry

Background:

  • Accurate calculation of molecular excited states is vital for understanding photochemistry and spectroscopy.
  • Traditional methods can be computationally expensive, limiting their application to complex systems.

Purpose of the Study:

  • To introduce a novel, efficient wave function ansatz for excited-state quantum Monte Carlo calculations.
  • To demonstrate the significance of orbital optimization in excited-state diffusion Monte Carlo (DMC).

Main Methods:

  • Excited-state variational Monte Carlo (VMC) and diffusion Monte Carlo (DMC) calculations.
  • A new wave function ansatz approximating configuration interaction singles with orbital relaxation.
  • Application of large augmented basis sets.

Main Results:

  • The proposed ansatz accurately approximates excited-state wave functions.
  • Demonstrated the critical role of orbital optimization in excited-state DMC.
  • Achieved results comparable to near-exact quantum chemical benchmarks.

Conclusions:

  • The developed method offers an efficient and accurate approach for excited-state calculations.
  • The study highlights the importance of orbital relaxation and optimization for diffuse excited states.
  • Successfully applied to various molecules including water, formaldehyde, formaldimine, and benzonitrile.