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Related Concept Videos

Chemical and Solubility Equilibria02:21

Chemical and Solubility Equilibria

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The free energy change associated with dissolving a solute in a liter of solvent is called the free energy of a solution, ΔGsolution. The overall ΔGsolution is expressed as the balance of ΔGinteraction against the always-favorable free-energy of mixing, ΔGmixing. Solution formation is favorable if  ΔGsolution is less than zero, whereas it is unfavorable if ΔGsolution is greater than zero. In short, for a solution to form and complete dissolution to take place,...
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Chemical Equilibria: Systematic Approach to Equilibrium Calculations01:21

Chemical Equilibria: Systematic Approach to Equilibrium Calculations

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Equilibrium calculations for systems involving multiple equilibria are often complex. For example, to calculate the solubility of a sparingly soluble salt in an aqueous solution in the presence of a common ion, one must consider all the equilibria in this solution. Calculations for these systems can be complicated and tedious, so a systematic approach with a series of steps is often helpful. The process is detailed below.
The first step is to identify all the chemical reactions involved, The...
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Calculating Equilibrium Concentrations02:05

Calculating Equilibrium Concentrations

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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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Calculating Standard Free Energy Changes02:49

Calculating Standard Free Energy Changes

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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...
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The Small x Assumption02:20

The Small x Assumption

45.8K
If a reaction has a small equilibrium constant, the equilibrium position favors the reactants. In such reactions, a negligible change in concentration may occur if the initial concentrations of reactants are high and the Kc value is small. In such circumstances, the equilibrium concentration is approximately equal to its initial concentration.  This estimation can be used to simplify the equilibrium calculations by assuming that some equilibrium concentrations are equal to the initial...
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Calculating the Equilibrium Constant02:46

Calculating the Equilibrium Constant

30.7K
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:
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Related Experiment Video

Updated: May 23, 2025

Excitonic Hamiltonians for Calculating Optical Absorption Spectra and Optoelectronic Properties of Molecular Aggregates and Solids
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ABCG2: A Milestone Charge Model for Accurate Solvation Free Energy Calculation.

Xibing He1, Viet H Man1, Wei Yang2

  • 1Department of Pharmaceutical Sciences and Computational Chemical Genomics Screening Center, School of Pharmacy, University of Pittsburgh, Pittsburgh, Pennsylvania 15261, United States.

Journal of Chemical Theory and Computation
|March 11, 2025
PubMed
Summary
This summary is machine-generated.

We developed ABCG2, a new atomic charge model, achieving chemical accuracy in molecular simulations. Combined with GAFF2, it accurately predicts solvation and transfer free energies for organic molecules.

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Area of Science:

  • Computational chemistry
  • Molecular modeling
  • Drug discovery

Background:

  • Accurate atomic charge models are crucial for molecular simulations.
  • Existing models may lack accuracy or transferability for diverse organic molecules.
  • The general AMBER force field (GAFF2) is a widely used force field for organic systems.

Purpose of the Study:

  • To develop and validate ABCG2, a novel charge model for organic molecules.
  • To assess the performance of ABCG2 in combination with GAFF2 for various thermodynamic properties.
  • To demonstrate the accuracy, transferability, and generality of the GAFF2/ABCG2 combination.

Main Methods:

  • Development of the ABCG2 charge model.
  • Validation against the FreeSolv database for hydration free energies.
  • Testing on the Minnesota Solvation Database for solvation and transfer free energies.
  • Benchmarking against densities of neat liquids and heats of vaporization.
  • Comparison with the restrained electrostatic potential (RESP) charge method.

Main Results:

  • Achieved RMSE of 0.99 kcal/mol for hydration free energy (FreeSolv), meeting chemical accuracy.
  • Obtained RMSE of 0.89 kcal/mol for solvation free energy and 0.85 kcal/mol for transfer free energies (Minnesota Solvation Database).
  • Demonstrated comparable performance to RESP for liquid densities and heats of vaporization.
  • Showed significantly smaller charge fluctuations compared to RESP for drug molecules.

Conclusions:

  • The GAFF2/ABCG2 combination provides accurate and reliable predictions of thermodynamic properties for organic molecules.
  • ABCG2 offers improved charge assignment stability over RESP.
  • The model demonstrates broad applicability and transferability in molecular simulations.