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The '''computation of phase equilibria''' using [[Computer simulation techniques |computer simulation]] can follow a number of different strategies. Here we will focus mainly on [[first-order transitions]] in fluid phases, usually [[Gas-liquid phase transitions |liquid-vapour]] equilibria.
Thermodynamic equilibrium implies, for two phases <math> \alpha </math> and <math> \beta </math>:
Thermodynamic equilibrium implies, for two phases <math> \alpha </math> and <math> \beta </math>:
* equal [[temperature]]s: <math> T_{\alpha} = T_{\beta} </math>
 
* equal [[pressure]]s: <math> p_{\alpha} = p_{\beta} </math>
* Equal [[temperature]]: <math> T_{\alpha} = T_{\beta} </math>
* equal [[chemical potential]]s: <math> \mu_{\alpha} = \mu_{\beta} </math>
 
* Equal [[pressure]]: <math> p_{\alpha} = p_{\beta} </math>
 
* Equal [[chemical potential]]: <math> \mu_{\alpha} = \mu_{\beta} </math>
 
The computation of phase equilibria using computer simulation can follow a number of different strategies. Here we will focus mainly
on first order transitions in fluid phases, usually liquid-vapor equilibria.
== Independent simulations for each phase at fixed temperature  in the [[canonical ensemble]]  ==
== Independent simulations for each phase at fixed temperature  in the [[canonical ensemble]]  ==
Simulations can be carried out  using either the [[Monte Carlo]] or the [[molecular dynamics]] technique.
Simulations can be carried out  using either the [[Monte Carlo]] or the [[molecular dynamics]] technique.
Assuming that one has some knowledge on the [[phase diagrams | phase diagram]] of the system, one can try the following recipe:
Assuming that one has some knowledge on the phase diagram of the system, one can try the following recipe:
# Fix a temperature and a number of particles
# Fix a temperature and a number of particles
#Perform a limited number of simulations in the low density region (where the gas phase density is expected to be)
#Perform a limited number of simulations in the low density region (where the gas phase density is expected to be)
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because is is not unusual have large uncertainties in the results for the properties.
because is is not unusual have large uncertainties in the results for the properties.
The basic idea is to use [[thermodynamic consistency]] requirements to improve the analysis.
The basic idea is to use [[thermodynamic consistency]] requirements to improve the analysis.
== Methodology in the [[Isothermal-isobaric ensemble|NpT]] ensemble ==
== Methodology in the [[Isothermal-isobaric ensemble|NpT]] ensemble ==
=== Low temperature: <math> \left. T << T_c \right. </math> ===
=== Low temperature: <math> \left. T << T_c \right. </math> ===
For temperatures well below the [[critical points | critical point]], provided that the calculation of the chemical potential
 
For temperatures well below the critical point, provided that the calculation of the chemical potential
of the liquid phase using [[Widom test-particle method]] gives precise results, the following strategy can be used to obtain a 'quick' result:
of the liquid phase using [[Widom test-particle method]] gives precise results, the following strategy can be used to obtain a 'quick' result:
#Perform an <math> NpT </math> simulation of the liquid phase at zero pressure, i.e. <math> p \simeq 0 </math>
#Perform an <math> NpT </math> simulation of the liquid phase at zero pressure, i.e. <math> p \simeq 0 </math>
#Arrive at an initial estimate, <math> \mu^{(1)} </math> for the coexistence value of the chemical potential by computing, in the liquid phase: <math> \left. \mu^{(1)} = \mu_l (N,T,p=0) \right. </math>
#Arrive at an initial estimate, <math> \mu^{(1)} </math> for the coexistence value of the chemical potential by computing, in the liquid phase: <math> \left. \mu^{(1)} = \mu_l (N,T,p=0) \right. </math>
#Make a first estimate of the coexistence pressure, <math> p^{(1)} </math>, by computing, either via simulation or via the [[Virial coefficients of model systems |virial coefficients]] of the gas phase, the pressure at which the gas phase fulfils: <math> \left. \mu_g(N,T, p^{(1)} ) = \mu^{(1)} \right. </math>
#Make a first estimate of the coexistence pressure, <math> p^{(1)} </math>, by computing, either via simulation or via the [[Virial coefficients of model systems |virial coefficients]] of the gas phase, the pressure at which the gas phase fulfills: <math> \left. \mu_g(N,T, p^{(1)} ) = \mu^{(1)} \right. </math>
#Refine the results, if required, by performing a simulation of the liquid phase at <math> \left.  p^{(1)} \right.  </math>, or use estimates of <math> \left( \partial V / \partial p \right)_{N,T,p=0} </math> (from the initial simulation) and the gas equation of state data to correct the initial estimates of pressure and chemical potential at coexistence.  
#Refine the results, if required, by performing a simulation of the liquid phase at <math> \left.  p^{(1)} \right.  </math>, or use estimates of <math> \left( \partial V / \partial p \right)_{N,T,p=0} </math> (from the initial simulation) and the gas equation of state data to correct the initial estimates of pressure and chemical potential at coexistence.  
Note that this method works only if the liquid phase remains metastable at zero pressure.
Note that this method works only if the liquid phase remains metastable at zero pressure.
=== Weak first order transitions ===
=== Weak first order transitions ===
There are situations where other strategies based on [[Isothermal-isobaric ensemble|NpT]] ensemble simulations can be used.
There are situations where other strategies based on [[Isothermal-isobaric ensemble|NpT]] ensemble simulations can be used.
Similar approaches have also been applied in the [[Monte Carlo in the grand-canonical ensemble|Grand Canonical]] ensemble.
Similar approaches have also been applied in the [[Monte Carlo in the grand-canonical ensemble|Grand Canonical]] ensemble.
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to cross from one phase to the other when it is being simulated. Somehow, the situation is now the opposite to that
to cross from one phase to the other when it is being simulated. Somehow, the situation is now the opposite to that
described in Section 2.1.
described in Section 2.1.
Taking into the account the classical partition function in the [[isothermal-isobaric ensemble|NpT]] ensemble it can be written:
Taking into the account the classical partition function in the [[isothermal-isobaric ensemble|NpT]] ensemble it can be written:


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conditions <math> \left. N,P,T \right. </math>. Let <math> p_{eq} </math> the pressure at which the phase transition occurs. In such a
conditions <math> \left. N,P,T \right. </math>. Let <math> p_{eq} </math> the pressure at which the phase transition occurs. In such a
case the following scenario is expected for <math> \left. P(V|N,p,T) \right. </math>:
case the following scenario is expected for <math> \left. P(V|N,p,T) \right. </math>:
*<math> \left. P(V|N,p_{eq},T) \right. </math> has two maxima, corresponding to the liquid and vapor pure phases, with <math> \left. P(V_v|N,p_{eq},T) = P(V_l|N,p_{eq},T) = P_{v/l} \right. </math>
*<math> \left. P(V|N,p_{eq},T) \right. </math> has two maxima, corresponding to the liquid and vapor pure phases, with
 
:: <math> \left. P(V_v|N,p_{eq},T) = P(V_l|N,p_{eq},T) = P_{v/l} \right. </math>


*The probability of a given intermediate volume at <math> \left. p_{eq} \right. </math>  can be estimated (from macroscopic arguments) as:
*The probability of a given intermediate volume at <math> \left. p_{eq} \right. </math>  can be estimated (from macroscopic arguments) as:


:<math> \left. P(V|N,p_{eq},T)  \simeq P_{v/l} \times \exp \left[ - \frac{ \gamma(T) \mathcal A }{k_B T } \right]          \right. </math>,
:: <math> \left. P(V|N,p_{eq},T)  \simeq P_{v/l} \times \exp \left[ - \frac{ \gamma(T) \mathcal A }{k_B T } \right]          \right. </math>,


where <math> \left. \gamma(T) \right. </math> is the [[surface tension]] of the vapor-liquid interface,  
where <math> \left. \gamma(T) \right. </math> is the [[surface tension]] of the vapor-liquid interface,  
and <math> \left. {\mathcal A} \right. </math> is the  
and <math> \left. {\mathcal A} \right. </math> is the  
surface area, which depends on the thermodynamic
surface area, which depends on the thermodynamic
variables <math> \left. (N,V,T) \right.  </math> and the geometry of the [[Periodic boundary conditions |simulation box]].  
variables <math> \left. (N,V,T) \right.  </math> and the geommetry of the simulation box.  
 
For small values of the surface tension, small system sizes and good simulation algorithms it could be possible for
For small values of the surface tension, small system sizes and good simulation algorithms it could be possible for
pressures close to <math> p_{eq} </math> to sample in a simulation the whole region of densities between
pressures close to <math> p_{eq} </math> to sample in a simulation the whole region of densities between
vapour and liquid densities. If such is the case the phase equilibria conditions can be computed
vapor and liquid densities. If such is the case the phase equilbria conditions can be computed
by reweighing techniques applied on the volume histograms of the simulation.
by reweighting techniques applied on the volume histogramas of the simulation.
==== Simple reweighing of the volume probability distribution ====
 
==== Simple reweighting of the volume probability distribution ====
Suppose that a precise <math> \left. NpT \right. </math> simulation has been carried out at pressure <math> \left. p_0 \right. </math>. From that simulation a volume probability distribution,  <math> \left. P_0(V|N,p_0,T) \right. </math>
Suppose that a precise <math> \left. NpT \right. </math> simulation has been carried out at pressure <math> \left. p_0 \right. </math>. From that simulation a volume probability distribution,  <math> \left. P_0(V|N,p_0,T) \right. </math>
has been computed. It is possible to use this function to estimate the distributions of volume for pressure values
has been computed. It is possible to use this function to estimate the distributions of volume for pressure values
close to <math> \left. p_0 \right. </math>;
close to <math> \left. p_0 \right. </math>;


: <math> \left. P(V|N,p,T) \propto \exp \left[ - \frac{ (p-p_0) V }{k_B T } \right] P_0(V|N,p_0,T) \right. </math>
:: <math> \left. P(V|N,p,T) \propto \exp \left[ - \frac{ (p-p_0) V }{k_B T } \right] P_0(V|N,p_0,T) \right. </math>


Using this reweighing procedure we can estimate the value of <math> \left. p_{eq} \right. </math>; that is: the value
Using this reweighting procedure we can estimate the value of <math> \left. p_{eq} \right. </math>; that is: the value
of pressure for which the volume distribution presents two peaks of equal height.
of pressure for which the volume distribution presents two peaks of equal height.
The analysis of the form of the distributions at the equilibrium conditions for different system sizes (i.e.
The analysis of the form of the distributions at the equilibrium conditions for different system sizes (i.e.
in the current case for different values of <math> \left. N \right. </math>) can be useful to distinguish between  
in the current case for different values of <math> \left. N \right. </math>) can be useful to distinghish between  
continuous and first order [[phase transitions]].  
continuous and first order [[phase transitions]].  
[Appropriate references should be quoted here]]--Noe 15:25, 2 October 2007 (CEST)
[This subsection is provisional, I have tho check a number of isssues]--Noe 10:49, 2 October 2007 (CEST)
[Working here]  --Noe 10:30, 2 October 2007 (CEST)
=== See also ===
=== See also ===
[[Surface tension]]
[[Surface tension]]
== Van der Waals loops in the [[canonical ensemble|canonical ]]  ensemble ==
== Van der Waals loops in the [[canonical ensemble|canonical ]]  ensemble ==
It is possible to compute the liquid-vapour equilibrium without explicit calculations of the chemical potential (or the pressure)
 
by performing a number of simulations sampling appropriately the vapour, liquid, and intermediate regions.
 
It is possible to compute the liquid-vapor equilibrium without explicit calculations of the chemical potential (or the pressure)
by performing a number of simulations sampling appropriately the vapor, liquid, and intermediate regions.
 
As an example, consider a simple fluid at a given subcritical temperature (<math> \left. T < T_c \right. </math>). We can perform a number of simulations for a given number of particles, <math> \left. N \right. </math> and different densities:
As an example, consider a simple fluid at a given subcritical temperature (<math> \left. T < T_c \right. </math>). We can perform a number of simulations for a given number of particles, <math> \left. N \right. </math> and different densities:


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In these simulations, we can compute the pressure (or the chemical potential) and fit the result to an appropriate equation.  
In these simulations, we can compute the pressure (or the chemical potential) and fit the result to an appropriate equation.  
With such an ''equation of state'' the phase equilibria can be estimated.
With such an ''equation of state'' the phase equilbria can be estimated.
 
If two phase equilibria exists, a ''loop'' in the representation of <math> \left. p = p (\rho) \right. </math> (or  <math> \left. \mu = \mu (\rho) \right. </math>)
If two phase equilibria exists, a ''loop'' in the representation of <math> \left. p = p (\rho) \right. </math> (or  <math> \left. \mu = \mu (\rho) \right. </math>)
should appear.
should appear.
* Computing <math> \left. \mu(\rho) \right. </math> from the equation of state given as <math> \left. p(\rho) \right. </math>:
* Computing <math> \left. \mu(\rho) \right. </math> from the equation of state given as <math> \left. p(\rho) \right. </math>:
For fixed temperature and number of particles:
For fixed temperature and number of particles:
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:<math> \left. A = -  p V + \mu N ; \; \;  \; a = - p/ \rho + \mu \right. </math>
:<math> \left. A = -  p V + \mu N ; \; \;  \; a = - p/ \rho + \mu \right. </math>


therefore:  
 
therefore: [I have to check the equations]--Noe 13:03, 26 September 2007 (CEST)
 


: <math> \left. \mu(\rho) - p(\rho)/\rho = \mu(\rho_0) - p(\rho_0)/\rho_0 + \int_{\rho_0}^{\rho} \frac{ p(\rho') }{(\rho')^2} d \rho' \right. </math>
: <math> \left. \mu(\rho) - p(\rho)/\rho = \mu(\rho_0) - p(\rho_0)/\rho_0 + \int_{\rho_0}^{\rho} \frac{ p(\rho') }{(\rho')^2} d \rho' \right. </math>
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A similar procedure can be built up to compute <math> \left. p(\rho) \right. </math>
A similar procedure can be built up to compute <math> \left. p(\rho) \right. </math>
from <math> \left. \mu(\rho) \right. </math>.
from <math> \left. \mu(\rho) \right. </math>.
Once <math> \left. p(\rho) \right. </math> and <math> \left. \mu(\rho) \right. </math> are known  it is straightforward to compute the coexistence point.
 
Once <math> \left. p(\rho) \right. </math> and <math> \left. \mu(\rho) \right. </math> are known  it is straighforward to compute the coexistence point.
==== Practical details ====
==== Practical details ====
Some precautions should be taken if this procedure is used:
Some precautions should be taken if this procedure is used:
* The precision of the simulation results in the two phase region will be poor (so, large simulations are required to have a good estimation of the equation of state)  
* The precision of the simulation results in the two phase region will be poor (so, large simulations are required to have a good estimation of the equation of state)  
*  The simulation results in the two phase region will depend dramatically on the system size (calculations with different number of particles become convenient to check the quality of the phase equilibria results)
*  The simulation results in the two phase region will depend dramatically on the system size (calculations with different number of particles become convenient to check the quality of the phase equilibria results)
== Direct simulation of the two phase system==
 
[[Image:direct_coexistence.png|300px|right]]
== Direct simulation of the two phase system in the [[canonical  ensemble]] ==
Direct simulation of the two phase system was first implemented by Abraham <ref>[http://dx.doi.org/10.1103/PhysRevB.23.6145 Farid F. Abraham "Two-dimensional melting, solid-state stability, and the Kosterlitz-Thouless-Feynman criterion", Physical Review B '''23''' pp. 6145-6148 (1981)]</ref>.
It has since been applied to the [[Lennard-Jones model]] <ref>[http://dx.doi.org/10.1063/1.1474581 James R. Morris and Xueyu Song "The melting lines of model systems calculated from coexistence simulations", Journal of Chemical Physics '''116''' 9352 (2002)]</ref>
and [[water]]
<ref>[http://dx.doi.org/10.1063/1.2183308 Ramón García Fernández, José L. F. Abascal, and Carlos Vega "The melting point of ice Ih for common water models calculated from direct coexistence of the solid-liquid interface", Journal of Chemical Physics '''124''' 144506 (2006)]</ref>.


== Gibbs ensemble Monte Carlo for one component systems==
== Gibbs ensemble Monte Carlo for one component systems==
The [[Gibbs ensemble Monte Carlo]] method is often considered as a smart variation of the standard canonical ensemble procedure (See <ref>[http://dx.doi.org/10.1080/00268978700101491 Athanassios Panagiotopoulos "Direct determination of phase coexistence properties of fluids by Monte Carlo simulation in a new ensemble", Molecular Physics '''61''' pp. 813-826 (1987)]</ref>).  
The [[Gibbs ensemble Monte Carlo]] method is often considered as a 'smart' variation of the standard canonical ensemble procedure (See Ref. 1).  
The simulation is, therefore, carried out at constant volume, temperature and number of particles.
The simulation is, therefore, carried out at constant volume, temperature and number of particles.
The whole system is divided into two non-interacting parts, each one has its own simulation
The whole system is divided into two non-interacting parts, each one has its own simulation
box with its own [[periodic boundary conditions]].
box with its own [[periodic boundary conditions]].
This separation of the two phases into different boxes is in order to suppress any influence due to  [[interface | interfacial]] effects.
This separation of the two phases into different boxes is in order to suppress any influence due to  interfacial effects.
The two subsystems can interchange volume and particles. The rules for these interchanges are
The two subsystems can interchange volume and particles. The rules for these interchanges are
built up so as to  guarantee  conditions of both chemical and mechanical equilibrium between  
built up so as to  guarantee  conditions of both chemical and mechanical equilibrium between  
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== Mixtures ==
== Mixtures ==
=== Symmetric mixtures ===
=== Symmetric mixtures ===
Examples of symmetric [[mixtures]] can be found both in lattice of continuous model. The [[Ising Models | Ising model]] can be
 
viewed as  mixture of two different chemical species which de-mix at low
temperature. The symmetry in the interactions can be exploited to simplify the calculation of phase diagrams.
== See also==  
== See also==  
*[[Constrained cell method]]
*[[Gibbs-Duhem integration]]
*[[Gibbs-Duhem integration]]
*[[Phase switch Monte Carlo]]
==References==
==References==
<references/>
#[http://dx.doi.org/10.1080/00268978700101491 Athanassios Panagiotopoulos "Direct determination of phase coexistence properties of fluids by Monte Carlo simulation in a new ensemble", Molecular Physics '''61''' pp. 813-826 (1987)]
[[category: computer simulation techniques]]
[[category: computer simulation techniques]]
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