//Create a spin lattice of given width, height, exchange coupling and external field
public IsingLattice(int p_width,
int p_height,
double p_ExchangeCoupling = 1,
double p_ThermalEnergy = 0,
double p_ExternalField = 0,
int p_StabilityLimit = 1)
{
last_spins = new PeriodicSpinLattice (p_width, p_height);
current_spins = new PeriodicSpinLattice (p_width, p_height);
exchange_coupling = p_ExchangeCoupling;
external_field = p_ExternalField;
thermal_energy = p_ThermalEnergy;
StabilityLimit = p_StabilityLimit;
random_generator = new Random ();
this.Randomize();
}
//Compute the probability of getting an up spin at a given position in a future state
//that is given by Exp(-energy_up/ThermalEnergy)/(Exp(-energy_down/ThermalEnergy) + Exp(-energy_up/ThermalEnergy))
//where energy_down = -energy_up
double ComputeUpSpinProbability(PeriodicSpinLattice current_lattice, int h_pos, int v_pos)
{
if (ThermalEnergy != 0) {
return 1 / (1 + Math.Exp (2 * ComputeEnergyPerSpin (current_lattice, h_pos, v_pos) / ThermalEnergy));
} else {
return (ComputeEnergyPerSpin (current_lattice, h_pos, v_pos) > 0) ? (0) : (1);
}
}
//Compute the next spin state at a given position of the lattice, in a probabilistic fashion
int ComputeFutureState(PeriodicSpinLattice current_lattice, int h_pos, int v_pos)
{
if (random_generator.NextDouble () < ComputeUpSpinProbability (current_lattice, h_pos, v_pos)) {
return +1;
} else {
return -1;
}
}
//Returns the energy per spin at a given position of the lattice: if multiplied by +1, gives
//the energy associated to spin state +1, and if multiplied by -1, gives the energy associated
//to spin state -1
double ComputeEnergyPerSpin(PeriodicSpinLattice current_lattice, int h_pos, int v_pos)
{
//Measure nearest neighbour spins with periodic boundary conditions
int left_spin = current_lattice [h_pos - 1, v_pos];
int right_spin = current_lattice [h_pos + 1, v_pos];
int top_spin = current_lattice [h_pos, v_pos - 1];
int bottom_spin = current_lattice [h_pos, v_pos + 1];
//Return the energy per spin associated to the requested spin state
return -ExchangeCoupling * (left_spin + right_spin + top_spin + bottom_spin) - ExternalField;
}
//Copy the contents of one spin lattice into another spin lattice
public bool CopyFrom(PeriodicSpinLattice reference)
{
//Make sure that the copy is actually possible
if(reference.Width != Width) return false;
if(reference.Height != Height) return false;
//Perform the data copy
for (int i=0; i < Width; ++i) {
for (int j=0; j < Height; ++j) {
this[i, j] = reference [i, j];
}
}
return true;
}
//Check if two spin lattices have identical contents
public bool Equals(PeriodicSpinLattice reference)
{
for (int i=0; i < Width; ++i) {
for (int j=0; j < Height; ++j) {
if (this[i, j] != reference[i, j])
{
return false;
}
}
}
return true;
}
