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Related Concept Videos

Free Energy Changes for Nonstandard States03:25

Free Energy Changes for Nonstandard States

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The free energy change for a process taking place with reactants and products present under nonstandard conditions (pressures other than 1 bar; concentrations other than 1 M) is related to the standard free energy change according to this equation:
 
where R is the gas constant (8.314 J/K·mol), T is the absolute temperature in kelvin, and Q is the reaction quotient. This equation may be used to predict the spontaneity of a process under any given set of conditions.
Reaction Quotient...
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Free Energy and Equilibrium00:55

Free Energy and Equilibrium

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The free energy change for a process may be viewed as a measure of its driving force. A negative value for ΔG represents a driving force for the process in the forward direction, while a positive value represents a driving force for the process in the reverse direction. When ΔG is zero, the forward and reverse driving forces are equal, and the process occurs in both directions at the same rate (the system is at equilibrium).
The reaction quotient, Q, is a convenient measure of the...
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Gibbs Free Energy and Thermodynamic Favorability02:23

Gibbs Free Energy and Thermodynamic Favorability

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The spontaneity of a process depends upon the temperature of the system. Phase transitions, for example, will proceed spontaneously in one direction or the other depending upon the temperature of the substance in question. Likewise, some chemical reactions can also exhibit temperature-dependent spontaneities. To illustrate this concept, the equation relating free energy change to the enthalpy and entropy changes for the process is considered:
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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...
21.3K
The Nernst Equation02:59

The Nernst Equation

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Nonstandard Reaction Conditions
The interconnection between standard cell potentials and various thermodynamic parameters such as the standard free energy change ΔG° and equilibrium constant K has been previously explored. For example, a redox reaction involving zinc(II) and tin(II) ions at 1 M concentration with Eºcell = +0.291 V and ΔG° = −56.2 kJ is spontaneous.
40.9K
Reaction Quotient02:35

Reaction Quotient

48.5K
The status of a reversible reaction is conveniently assessed by evaluating its reaction quotient (Q). For a reversible reaction described by m A + n B ⇌ x C + y D, the reaction quotient is derived directly from the stoichiometry of the balanced equation as
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Updated: Jul 5, 2025

An Analog Macroscopic Technique for Studying Molecular Hydrodynamic Processes in Dense Gases and Liquids
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Computing equilibrium free energies through a nonequilibrium quench.

Kangxin Liu1,2, Grant M Rotskoff3, Eric Vanden-Eijnden4

  • 1Department of Chemistry, New York University, New York, New York 10003, USA.

The Journal of Chemical Physics
|January 19, 2024
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This study implements a novel "quench" dynamics method for molecular simulations, efficiently calculating free energy surfaces (FESs) by combining high-temperature sampling with rapid cooling. The approach accurately models systems like alanine dipeptide, especially when integrated with umbrella sampling for comprehensive results.

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Area of Science:

  • Computational Chemistry
  • Statistical Mechanics
  • Molecular Dynamics

Background:

  • Accelerating molecular simulations often involves temperature manipulation for rare transitions.
  • Existing methods use reweighting or Monte Carlo exchanges at higher temperatures.
  • A prior study proposed nonequilibrium quench dynamics for density of states calculations.

Purpose of the Study:

  • Implement and evaluate quench dynamics in LAMMPS for molecular systems.
  • Develop and test nonequilibrium estimators for partition functions and free energy surfaces (FESs).
  • Assess the accuracy and efficiency of the quench method compared to traditional approaches.

Main Methods:

  • Implementation of quench dynamics in the LAMMPS molecular dynamics package.
  • Development of novel nonequilibrium estimators for free energy calculations.
  • Testing on a minimal model of harmonic springs and the alanine dipeptide system.
  • Comparison with reference umbrella sampling calculations.

Main Results:

  • The quench method is exact for independent harmonic springs.
  • It provides accurate FES near stable configurations for alanine dipeptide.
  • Combining quench dynamics with umbrella sampling enables efficient FES calculation across all regions.
  • The combined scheme allows for cost-free FES computation at multiple temperatures.

Conclusions:

  • The quench dynamics approach, especially when combined with umbrella sampling, offers a highly efficient method for calculating molecular free energy surfaces.
  • This integrated method overcomes limitations of standard umbrella sampling, particularly for obtaining FES across various temperatures.
  • The approach shows promise for extensions, such as solute tempering, demonstrating high accuracy for solvated systems.