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Poisson Boltzmann eq. with an infinite boundary condition

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Roi Bar-On
Roi Bar-On on 22 Oct 2020
Commented: Roi Bar-On on 3 Nov 2020 at 22:35
Hi everybody!
I've written a code that solves the following PB problem
This is a 2nd order ode and the dependant variable here is psi (tilde) and the independant variable is x (tilde). The boundary conditions are:
The code includes an optimizer based on the secant method. I'm trying to guess the solution that will ''land'' on the 2nd boundary condition via the shooting method:
function [x2,p2]=optimumup(y21,y22,bc2,l)
%optimizing the choice of y2 for the shooting method, using the Newton
%method (secant) by solving the equation p(length(p(:,1))-bc2=0
[x1,p1] = BP(y21,l);
[x2,p2] = BP(y22,l)
for i=1:10
y23=y22-(p2(length(p2(:,1)),1)-bc2)/dp;%recurrence relation
[x2,p2] = BP(y23,l);
if abs(p2(length(p2(:,1)))-bc2)<0.001;
disp('No success')
function [x,p] = BP(y2,l)
%Shooting method for the BP eq
options = odeset('RelTol',1e-4,'AbsTol',1e-4);
[x,p] = ode45(@eqs,[0 l],[90/24 y2],options);
function [dy] = eqs(x,y)
%PB equation
dy = zeros(2,1); % a column vector
dy(1) = y(2);
dy(2) = sinh(y(1));
y21 and y22 are the initial x guesses for the secant method. bc2 is the boundary condition at x goes to inifinity. l is the interval size.
I have two main problems:
  1. I don't know how to incorperate the inifinite boundary condition.
  2. For some reason my solution does not converge. I'm working dimensionless.
I would appreciate some help guys.


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Accepted Answer

Alan Stevens
Alan Stevens on 23 Oct 2020
I suspect the initial gradient required to get zero at infinity is an irrational number (or possibly a rational number with a large number of digits in its decimal expansion!). See the following for example:
xf = 25;
xspan = [0 xf];
dydx0 = -1.042119689394;
dx = 10^-11;
dydx0hi = dydx0+dx;
dydx0lo = dydx0-dx;
Y0 = [1, dydx0];
[x,Y] = ode45(@eqs,xspan,Y0);
psi = Y(:,1);
Y0hi = [1, dydx0hi];
[xhi,Yhi] = ode45(@eqshi,xspan,Y0hi);
psihi = Yhi(:,1);
Y0lo = [1, dydx0lo];
[xlo,Ylo] = ode45(@eqslo,xspan,Y0lo);
psilo = Ylo(:,1);
axis([0 xf -2 2])
lbl1 = ['d\psi dx(x=0) = ',num2str(dydx0,12)];
lbl2 = ['d\psi dx(x=0) = ',num2str(dydx0hi,12)];
lbl3 = ['d\psi dx(x=0) = ',num2str(dydx0lo,12)];
function dY = eqs(~,Y)
%PB equation
dY = [Y(2);
function dY = eqshi(~,Y)
%PB equation
dY = [Y(2);
function dY = eqslo(~,Y)
%PB equation
dY = [Y(2);
This produces the following graph:
Although it might look as though the black line has converged on zero, if you extend the end time to, say 30, you will see it start to diverge.


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Alan Stevens
Alan Stevens on 24 Oct 2020
In effect I used the shooting method, except that I did it manually rather than writing a routine to do it! It’s actually not difficult, and took much less time than I would have taken to write a routine. Had the result looked as though the curve was tending to zero in a stable manner, I would then have written a routine to do it more accurately. However, as you can see from the fact that small perturbations in the initial slope lead to divergences in opposite directions, it seems that the final equilibrium is likely to be unstable.
A boundary condition at infinity suggests an asymptotic approach to zero (in this case). In physical systems, such as fluid flow boundary layers, for example, it is a convenient fiction standing in for a finite distance, so can often be replaced by a (relatively) large but finite value. If that is the case for you then you might be able to make use of the result. However, your equation doesn’t look as though it represents a physical system (I could be wrong!), so you will have to decide if it is useful or not.
Roi Bar-On
Roi Bar-On on 24 Oct 2020
Physically it describes the potential between two surfaces where one has a dimensionless potential of 1 and the other one has the potential of zero. As a matter a fact I'm solving almost similar problem with the same boundary conditions so i need a robust algorithm to work here.. I am seeking to incorporate my infinite boundary condition within the initial guess in the code as I mentioned.
Alan Stevens
Alan Stevens on 24 Oct 2020
So why do you need to set an infinite boundary condition, rather than use the distance between the surfaces?

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More Answers (3)

David Goodmanson
David Goodmanson on 24 Oct 2020
Edited: David Goodmanson on 24 Oct 2020
Hi Roi
For convenience I will use u in place of psi. An exact solution is
tanh(u/4) = A*exp(-x) where A = tanh(1/4) [1]
u = 4*atanh(A*exp(-x))
which meets both of the boundary conditons. For du/dx at the origin, it turns out that
du/dx = -2*sinh(1/2) = -1.042190610987495
which disagrees slightly from what Alan has, for reasons not known to me.
start with
u'' = sinh(u)
multiply by integrating factor u' and integrate
u'^2/2 = cosh(u) (+ constant)
u'^2 = 2*cosh(u) + constant
as x --> inf you want u --> 0 which also means that u' --> 0, so the constant = -2
u'^2 = 2*(cosh(u)-1) = 4*sinh(u/2)^2
u' = -2*sinh(u/2) % pick the negatatve root
du/(2*sinh(u/2)) = -dx
% integrate
log(tanh(u/4)) = -x + constant
tanh(u/4) = A*exp(-x)
you want u = 1 at x = 0 so
A = tanh(1/4)
For the derivative at the origin, take the derivative of both sides of [1] at the top
( (1/4) / cosh^2(u/4) ) du/dx = -A*exp(-x)
% evaluate this at x = 0 and u(0) = 1
( (1/4) / cosh^2(1/4) ) du/dx = -A
du/dx = -tanh(1/4)*4*cosh^2(1/4) = -4*sinh(1/4)*cosh(1/4) = -2*sinh(1/2)


David Goodmanson
David Goodmanson on 24 Oct 2020
Hi Alan,
Thanks, it did work in this special case but I think your comments about a finite boundary and/or an asymptotic approach are going to be more useful in general. In this case out at infinity the answer has to be u = A*exp(-x), so both u and u' are known there. Then starting at large x, integrating to the origin and using the shooting method for various A should also work, I think. I have not tried it.

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Roi Bar-On
Roi Bar-On on 24 Oct 2020
Because my problem is for a half space by definition..
I don't mind using a finite value for ''infinity'', it's okay. In the case you've presented x=5 is infinity and that is okay. the thing is that in my code as I attached, even if I'll take x=5 as ''infinity'', the solution won't converge. Have you tried to run it and got something maybe?

Roi Bar-On
Roi Bar-On on 25 Oct 2020 at 15:16
Thank you very much guys!
Analytical solution is great!
As for the numerical one with ode45 that I have attached. How can I guess the two x's that secant method requires correctly in order to converge to the same analytical solution?


Show 5 older comments
Roi Bar-On
Roi Bar-On on 30 Oct 2020 at 21:58
This is because I've demanded the inifnite bc to be zero? dpnew corresponds to this? I'm still a little bit confused, sorry.
Alan Stevens
Alan Stevens on 30 Oct 2020 at 22:32
Yes, one of your boundary conditions is that psi(x=infinity) = 0. So we try a value of dpsi/dx(x=0) and see what value of psi(x = infinity) we get. If the latter isn't zero, we try another value of dpsi/dx(x=0) and keep repeating this process until we hit on a value that makes psi(x=infintiy) zero. Instead of trying random guesses for dpsi/dx(x=0), the secant method provides a way of improving subsequent guesses (though the method isn't infallible!).

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