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Monte Carlo Simulation technique
S. B. Santra
Department of Physics
Indian Institute of Technology Guwahati

Introduction
• What is Monte Carlo (MC)?
Monte Carlo method is a common name for a wide variety of stochastic
techniques. These techniques are based on the use of random numbers (sampling)
and probability statistics to investigate problems in areas as diverse as economics,
Material Science, Statistical Physics, Chemical and Bio Physics, nuclear physics,
flow of traffic and many others. In general, to call something a "Monte Carlo"
method, all you need to do is use random numbers to examine your problem.
Why MC? Difficult to solve analytically, may have too many particles in the system,
may have complex interactions among the particles and or with the external field.

A simple example of MC
Can we determine the value of π using a
Monte Carlo method ?
Draw a square on the ground, then inscribe a
circle within it. Uniformly scatter some objects
of uniform size (grains of rice or sand) over the
square. Count the number of objects inside the
circle and the total number of objects. The ratio
of the two counts is an estimate of the ratio of
the two areas, which is π/4. Multiply the result
by 4 to estimate π.

What have we done?
1. Defined a domain of possible inputs.
2. Generated inputs randomly from a probability distribution over the domain.
3. Performed a deterministic computation on the inputs.
4. Aggregate the results.
Common properties of Monte Carlo methods:
The inputs should truly be random. If grains are purposefully dropped into only the
center of the circle, they will not be uniformly distributed, and so our approximation
will be poor.
There should be a large number of inputs. The approximation will generally be poor
if only a few grains are randomly dropped into the whole square. On average, the
approximation improves as more grains are dropped.
Domain of inputs is the square that circumscribes the circle.

Evaluation of definite Integral by MC
Very effective in evaluating higher dimensional integrations. Error is less in MC

How to make things random ?
• You need to have Random Number Generator.
• RNG provides uniformly distributed numbers between 0 and 1.
• How to generate?
– Congruential method: a simple and popular algorithm
In 32-bit linear congruential algorithm, a0 is zero, X0 is taken as an odd
number, c=16807, Nmax=231-1=2147483641.
provides numbers between 1 and Nmax.

Different types of MC
Classical Monte Carlo: samples are drawn from a probability distribution, often the
classical Boltzmann distribution, to obtain thermodynamic properties or minimum-
energy structures;
Quantum Monte Carlo: random walks are used to compute quantum-mechanical
energies and wave functions, often to solve electronic structure problems, using
Schrödinger's equation as a formal starting point;

Macroscopic system: Classical MC
Consider a macroscopic system described by a Hamiltonian H in thermodynamic
equilibrium with a heat bath at temperature T.
The expectation value of a physical quantity A is given by
If the system is described by discrete energy states
•One needs to calculate Z, the partition function, which gives an exact description of the
system.
•Evaluation of an infinite series or an integration over a 6N dimensional space.
•In most of the cases, it is not possible to evaluate Z exactly.
•Difficulty is not only in evaluating the integral/infinite series but also handling the
complex interaction appearing in the exponential.
=
1 ⁄

Monte Carlo sampling
• In general, it is not possible to evaluate the summation over such a large
number of state or the integral in such a high dimensional space.
• It is always possible to take average over a finite number of states, a subset
of all possible states, if one knows the correct Boltzmann weight of the
states and the probability with which they have to be picked up.
• Say, a small subset of all possible states are chosen at random with certain
probability distribution ps. So the best estimate of a physical quantity A will
be
Usually, in a MC simulation N is taken as 106 to 108.
= 2 , 2 ≈ 10
For a two state system:

Simple sampling Monte Carlo
• The problem is how to specify ps so that the chosen N states will lead to
right <A>?
• A simple choice of ps=p for all state will lead the estimator
Such average is generally suitable for non-thermal, non-interacting systems.
For these systems, T is taken constant but high.

Example-1: Random walk - Diffusion
=
1 2 ~ = 1 2
⁄
= − + −
= = 4.
For square lattice, For = 400, No. of configurations ≈ 10

Example-2: Self-avoiding walk- Linear
polymers in dilute solution
For = 2, = = 3 4
⁄

Percolation– disordered systems
Composite material
Composite material= mixture of metal and non-metal,
C is the concentration of conducting elements,
I is the current flowing in the circuit.
C
C*
(0,0)
I
1
1
C<C*
Non-conducting
C>C*
Always conducting*
C=C*
Just conducting/
Percolation

Other Examples
Dilute magnet: Paramagnet disordered by non-magnetic ions.
Concentration of magnetic ions is, say p.
p<pc
Non-magnetic
p>pc
Magnetic
Polymer gelation: Concentration of monomers is p.
p<pc
Solution
p>pc
Gel
Flow of liquid in porous media Local /extended wetting
Spread of a disease in a
population
Containment/epidemic
Stochastic star formation in
spiral galaxies
Non-propagation/propagation
Quarks in nuclear matter Confinement/non-confinement

p=0.4
p=0.6
p=0.8
Percolation model

p = 0.55 p = 0.58
p = 0.59274621 p = 0.65
Percolation clusters at different occupation probability

Moments of cluster size distribution and Connectivity length:
~ − ~ −
Signature of a second order phase transition

Importance sampling MC:
The statistical average of a quantity A is given by:
Sum is over all possible states.
At high temperature, the Boltzmann probability goes to 1 and it leads to
simple average.
However, in the low temperature limit, the system spends almost all its
time sitting in the ground state or one of the lowest excited states.
An extremely restricted part of the phase space should contribute to the
average and the sum should be over the few low energy states.
Picking up points randomly over the whole phase space is no good here.

Importance Sampling
It is a method that will lead us automatically in the important region of phase space.
How one could sample points preferentially from the region which is populated by
the appropriate states? Such a method is realizable via the importance sampling
MC scheme.
In such a scheme, a state s is picked up with a probability ps proportional to the
Boltzmann factor.
A MC estimator then can be defined for a subset of N microstates (much smaller
than all possible states) as
Note that ps=p leads to simple sampling

Importance Sampling
Instead of picking N states randomly, pick them with a probability ps, the
correct Boltzmann weight.
This averaging is called ‘importance sampling’ average.
Hoverer, it is not a easy task to pick up states which appear with its correct
Boltzmann weight.

Markov chain
It is possible to realize importance sampling with the help of Markov chain.
A Markov chain is a sequence of states each of which depends only on the
preceding one.
The transition probability Wnm from state n to m should not vary over time
and depends only on the properties of current states (n,m).
The Markov chain has the property that average over a successive
configurations
if the states are weighted by Boltzmann factor.

Master equation:
+ 1 = + − ( )
At time t:
State m
State n
( ) ( )
…..
…..

Ergodicity
It should be possible in Markov process to reach any state of the system starting
from any other state in long run.
This is necessary to achieve a state with its correct Boltzmann weight. Since each
state m appears with some non-zero probability pm in the Boltzmann distribution,
and if that state is inaccessible from another state n, then the probability to find the
state m starting from state n is zero. This is in contradiction with Boltzmann
distribution which demands the state should appear with a probability pm.
This means that there must be at least one path of non-zero transition probabilities
between any two states.
One should take care in implementing MC algorithms that it should not violate
ergodicity.

Detailed balance
In the steady state, the probability flux out of a state n is equal to the
probability flux into the state n and the steady state condition is given by the
“global balance” equation
However, the above condition is not sufficient to guarantee that the probability
distribution will tend to pn after a long evolution of a Markov chain starting from
any state.
Detailed balance:

Metropolis algorithm
Using this transition probability one cane now generate a series of states with their right
Boltzmann weight.
=
1 ⁄
=
1 ⁄
= ⁄
and where
Since the canonical partition function Z is extremely difficult to calculate exactly,
or is not known exactly.
= = ⁄
= −
where
Algorithm: =
⁄
> 0
1 ≤ 0

How to implement ? = ⁄
for > 0?
Call a random number = 0,1 uniformly distributed between 0 and 1.
If ≤ , change the state otherwise do not change the state.
Pnm
Enm
Enm
m
n

Application to Ising Model:
Set B=0.
The critical temperature:
= 15
⇒
Critical exponents:
Scaling:
~ ⁄
=
−
• Para to ferromagnetic transition
• Liquid gas transition
• Binary alloys
Applications:

Metropolis algorithm for spin-1/2 Ising model
1. Choice of initial state: Take a (L × L) square lattice. A random spin configuration is taken as
initial state assigning s = +1 (or −1) to the lattice sites randomly with probability 0.5
corresponding to up spin (or down spin) respectively.
2. Calculation of energy change: Choose a site j randomly from the sites with s = +1 or −1
which correspond to the initial state of the jth spin. The final state is the state with the
overturned spin at the jth site. The change in energy ∆ = − for the sing Hamiltonian
without external field is given by
where sj is the initial spin state and sk s are the nearest neighbour spins.
3. Accept or reject the spin flip: If ∆E < 0 flip the spin, else call a random number r uniformly
distributed between 0 and 1. If flip the spin, otherwise do not flip it.
4. Iteration: Go to the next site and start from (2). Repeat the process for all spins.

Calculation of change in energy: = −

Magnetization and Susceptibility:
The magnetization m per spin in a state n is given by
Since only one spin (k) flips at a time in the Metropolis algorithm, so the change in
magnetization is given by
One can then calculate the magnetization at the beginning of the simulation and then use
the following equation
One could calculate the per spin susceptibility as
=
∆ = − = − = − = 2
= + ∆ = + 2 =
1
×
=
1
−
= −

Magnetization and Susceptibility:
On a 2d square lattice of size 100 ×100.

Energy and specific heat:
= − Set J=1
One may calculate the per spin specific heat as
The energy difference in going from state n to state m is:

Time auto correlation
In order to take average of a physical quantity, we have presumed that the states over which
the average has been made are independent. Thus to make sure that the states are
independent, one needs to measure the “correlation time” τ of the simulation. The time auto
correlation Cm(t) of magnetization is defined as
The auto correlation Cm(t) is expected to fall off exponentially at long time
Thus, at t = τ , Cm(t) drops by a factor of 1/e from the maximum value at t = 0. For independent
samples, one should draw them at an interval greater than τ . In most of the definitions of
statistical independence, the interval turns out to be 2τ .

Time auto correlation
C
m
(t)
Critical slowing down

Cluster update algorithms
Cluster update algorithms are the most successful global update methods in use.
These methods update the variables globally , in one step, whereas the standard local
methods operate on one variable at a time
A global update can reduce the autocorrelation time of the update and thus greatly
reduce the statistical errors.
Two famous algorithms: Swendsen-Wang and Wolff

Swendsen-Wang algorithm: PRL 58 (1987) 86.
Beginning with an arbitrary configuration si, one SW cluster update cycle is:

Wolff single cluster algorithm: PRL 62 (1989) 361
Principle: do the cluster decomposition as in S-W , but invert (‘flip’) only one randomly chosen
cluster! In practice:

Auto correlation in cluster algorithm

Effect of finite size
Critical temperature:
Thermodynamic quantities: At = :

Wang-Landau
Instead of configuration sum, one may sum over the energy levels with right g(E).
Again it is difficult to calculate value of g(E) for large systems.
In conventional MC, one directly generate the canonical distribution:
In WL, g(E) is estimated directly and accurately by performing a random in energy
space. Change the states with a probability that depends on the density of states as
given below:

Kinetic Monte Carlo
With MD we can only reproduce the dynamics of the system for ≤ 100 ns. Slow thermally
activated processes, such as diffusion, cannot be modeled. Metropolis Monte Carlo samples
configurational space and generates configurations according to the desired statistical-mechanics
distribution. However, there is no time in Metropolis MC and the method cannot be used to study
evolution of the system or kinetics. An alternative computational technique that can be used to
study kinetics of slow processes is the kinetic Monte Carlo (kMC) method.
As compared to the Metropolis Monte Carlo method, kinetic Monte Carlo method has a
different scheme for the generation of the next state. The main idea behind kMC is to use
transition rates that depend on the energy barrier between the states, with time increments
formulated so that they relate
In Metropolis MC methods we decide whether to accept a move by considering the energy
difference between the states. In kMC methods we use rates that depend on the energy
barrier between the states.

Hybrid Monte Carlo
A hybrid Monte Carlo move involves running a molecular dynamics simulation for a
fixed length of time. The configuration at the end of this molecular dynamics run is
accepted or rejected based on the Metropolis criterion, where E(n) is the potential
energy before the molecular dynamics run, and E(m) is the potential energy at the
end of the run. Thus, molecular dynamics can be used to propose a Monte Carlo
move that involves the collective motion of many particles. An advantage that this
method has over conventional molecular dynamics simulations is that large time
steps may be used in the hybrid Monte Carlo move.
State m
State n
En
Em
MD

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