Interface pinning calculations: Difference between revisions

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'''Interface pinning'''{{cite|pedersen:prb:13}} is used to determine the melting point from a [[:Category: Molecular dynamics|molecular-dynamics]] simulation of the interface between a liquid and a solid phase.  
'''Interface pinning''' uses the <math>Np_zT</math> ensemble where the barostat only acts along the <math>z</math> direction.
<!-- == Theory == -->
This ensemble uses a Langevin thermostat and a Parrinello-Rahman barostat with lattice constraints in the remaining two dimensions.
The typical behavior of such a simulation is to freeze or melt, while the interface is ''pinned'' with a bias potential.
The solid-liquid interface must be in the <math>x</math>-<math>y</math> plane perpendicular to the action of the barostat.
This potential applies an energy penalty for deviations from the desired two-phase system.
It is preferred simulating above the melting point because the bias potential prevents melting better than freezing.


The Steinhardt-Nelson{{cite|steinhardt:prb:83}} order parameter <math>Q_6</math> discriminates between the solid and the liquid phase.
Set the following tags for the '''interface pinning''' method:
With the bias potential
;{{TAG|OFIELD_Q6_NEAR}}: Defines the near-fading distance <math>n</math>.
;{{TAG|OFIELD_Q6_FAR}}: Defines the far-fading distance <math>f</math>.
;{{TAG|OFIELD_KAPPA}}: Defines the coupling strength <math>\kappa</math> of the bias potential.
;{{TAG|OFIELD_A}}: Defines the desired value of the order parameter <math>A</math>.


:<math>U_\text{bias}(\mathbf{R}) = \frac\kappa2 \left(Q_6(\mathbf{R}) - A\right)^2 </math>
The following example {{TAG|INCAR}} file calculates the interface pinning in sodium{{cite|pedersen:prb:13}}:
 
{{TAGBL|TEBEG}} = 400                  # temperature in K
penalizes differences between the order parameter for the current configuration <math>Q_6({\mathbf{R}})</math> and the one for the desired interface <math>A</math>.
{{TAGBL|POTIM}} = 4                    # timestep in fs
<math>\kappa</math> is an adjustable parameter determining the strength of the pinning.
{{TAGBL|IBRION}} = 0                    # run molecular dynamics
 
{{TAGBL|ISIF}} = 3                      # use Parrinello-Rahman barostat for the lattice
Under the action of the bias potential, the system equilibrates to the desired two-phase configuration.
{{TAGBL|MDALGO}} = 3                    # use Langevin thermostat
An important observable is the difference between the average order parameter <math>\langle Q_6\rangle</math> in equilibrium and the desired order parameter <math>A</math>.
{{TAGBL|LANGEVIN_GAMMA_L}} = 3.0        # friction coefficient for the lattice degree of freedoms (DoF)
This difference relates to the the chemical potentials of the solid <math>\mu_\text{solid}</math> and the liquid <math>\mu_\text{liquid}</math> phase
{{TAGBL|LANGEVIN_GAMMA}} = 1.0          # friction coefficient for atomic DoFs for each species
 
  {{TAGBL|PMASS}} = 100                  # mass for lattice DoFs
:<math>
{{TAGBL|LATTICE_CONSTRAINTS}} = F F T  # fix x-y plane, release z lattice dynamics
N(\mu_\text{solid} - \mu_\text{liquid}) =  
  {{TAGBL|OFIELD_Q6_NEAR}} = 3.22        # near fading distance for function w(r) in Angstrom
\kappa (Q_{6,\text{solid}} - Q_{6,\text{liquid}})(\langle Q_6 \rangle - A)
{{TAGBL|OFIELD_Q6_FAR}} = 4.384        # far fading distance for function w(r) in Angstrom
</math>
{{TAGBL|OFIELD_KAPPA}} = 500            # strength of bias potential in eV/(unit of Q)^2
 
{{TAGBL|OFIELD_A}} = 0.15              # desired value of the Q6 order parameter
where <math>N</math> is the number of atoms in the simulation.
 
Computing the forces requires a differentiable <math>Q_6(\mathbf{R})</math>.
<!-- PLEASE REPHRASE - I did not understand this part and how it relates to Q_6(R) -->
In the VASP implementation a smooth fading function <math>w(r)</math> is used to weight each pair of atoms at distance <math>r</math> for the calculation of the <math>Q_6(\mathbf{R},w)</math> order parameter. This fading function is given as
 
:<math> w(r) = \left\{ \begin{array}{cl} 1  &\textrm{for} \,\, r\leq n \\
                      \frac{(f^2 - r^2)^2 (f^2 - 3n^2 + 2r^2)}{(f^2 - n^2)^3} &\textrm{for} \,\, n<r<f \\
                      0 &\textrm{for} \,\,f\leq r \end{array}\right. </math>
 
<!-- is w(r) equivalent to (1 - t)^2(1 + 2t) with t = (r - n) / (f - n)? -->
 
Here <math>n</math> and <math>f</math> are the near- and far-fading distances, respectively.
<!-- END REPHRASE -->
The radial distribution function <math>g(r)</math> of the crystal phase yields a good choice for the fading range.
To prevent spurious stress, <math>g(r)</math> should be small where the derivative of <math>w(r)</math> is large.
Set the near fading distance <math>n</math> to the distance where <math>g(r)</math> goes below 1 after the first peak.
Set the far fading distance <math>f</math> to the distance where <math>g(r)</math> goes above 1 again before the second peak.


== References ==
== References ==

Revision as of 11:48, 16 October 2024

Interface pinning uses the ensemble where the barostat only acts along the direction. This ensemble uses a Langevin thermostat and a Parrinello-Rahman barostat with lattice constraints in the remaining two dimensions. The solid-liquid interface must be in the - plane perpendicular to the action of the barostat.

Set the following tags for the interface pinning method:

OFIELD_Q6_NEAR
Defines the near-fading distance .
OFIELD_Q6_FAR
Defines the far-fading distance .
OFIELD_KAPPA
Defines the coupling strength of the bias potential.
OFIELD_A
Defines the desired value of the order parameter .

The following example INCAR file calculates the interface pinning in sodium[1]:

TEBEG = 400                   # temperature in K
POTIM = 4                     # timestep in fs
IBRION = 0                    # run molecular dynamics
ISIF = 3                      # use Parrinello-Rahman barostat for the lattice
MDALGO = 3                    # use Langevin thermostat
LANGEVIN_GAMMA_L = 3.0        # friction coefficient for the lattice degree of freedoms (DoF)
LANGEVIN_GAMMA = 1.0          # friction coefficient for atomic DoFs for each species
PMASS = 100                   # mass for lattice DoFs
LATTICE_CONSTRAINTS = F F T   # fix x-y plane, release z lattice dynamics
OFIELD_Q6_NEAR = 3.22         # near fading distance for function w(r) in Angstrom
OFIELD_Q6_FAR = 4.384         # far fading distance for function w(r) in Angstrom
OFIELD_KAPPA = 500            # strength of bias potential in eV/(unit of Q)^2
OFIELD_A = 0.15               # desired value of the Q6 order parameter

References