Particles in a 2D box - rounding errors when calculating the energy - c++

Question

I am trying to calculate distances between particles in a box. If the distance calculated is greater than a preset cut-off distance, then the potential energy is 0. Otherwise, it is 1.
There are some rounding issues I think and I am not familiar with variable types and passing variables through functions to know what to do next.
The error
When I calculate d0 by hand I get d0 = 0.070 - this is not what the computer gets! The computer gets a number on the order of e-310.
All of the calculated distances (dij) are no shorter than 1/14, which is much larger than e-310. According to my if statement, if dij>d0, then U=0, so I should get a total energy of 0, but this is what I get:
d0 is 6.95322e-310
i is 0 j is 1 dij is 0.0714286 d0 is 6.95322e-310 Uij is 1
.....
Energy of the system is 24976
Please let me know if I could provide any more information. I did not include the entirety of my code, but the other portion involves no manipulation of d0.
I copied the relevant pieces of code below
Part 1: relevant box data
class Vector {
public:
double x;
double y;
Vector() {
}
Vector (double x_, double y_) {
x = x_;
y = y_;
}
double len() {
return sqrt(x*x + y*y);
}
double lenSqr() {
return x*x + y*y;
}
};
class Atom
{
public:
Vector pos;
Vector vel;
Vector force;
Atom (double x_, double y_) {
pos = Vector(x_, y_);
}
};
class BoxData
{
public:
const double Len = 1.;
const double LenHalf = 0.5 * Len;
long double d = 1. / 14; // d is the distance between each atom
in the initial trigonal lattice
int nu = 7; // auxillary parameter - will be varied
long double d0 = d * (1 - 2^(nu - 8)); // cutoff distance
double alpha = d - d0; // maximum allowed displacement
};
int main() {
// Initialize box
LoadBox();
// Institute a for loop here
SystemEnergy();
MonteCarloMove();
return 0;
}
//Putting atoms into box
void LoadBox()
{
ofstream myfile("init.dat", ios::out);
//Load atoms in box in triangular offset lattice
const double x_shift = 1. / 14;
const double y_shift = 1. / 16;
double x = 0;
double y = 0;
double x_offset = 0;
for (y = 0; y <= 1. - y_shift; y += y_shift) {
for (x = x_offset; x < 0.99; x += x_shift) {
// create atom in position (x, y)
// and store it in array of atoms
atoms.push_back(Atom(x, y));
}
// every new row flip offset 0 -> 1/28 -> 0 -> 1/28...
if (x_offset < x_shift / 4) {
x_offset = x_shift / 2;
} else {
x_offset = 0.0;
}
}
const int numAtoms = atoms.size();
//print the position of each atom in the file init.dat
for (int i = 0; i < numAtoms; i++) {
myfile << "x is " << atoms[i].pos.x << " y is " << atoms[i].pos.y << endl;
}
myfile.close();
}
Part 2 : Energy calculation
vector<Atom> atoms;
BoxData box_;
void SystemEnergy()
{
ofstream myfile("energy.dat", ios::out);
double box_Len, box_LenHalf, box_d0;
double dij; // distance between two atoms
double Uij; // energy between two particles
double UTotal = 0;
double pbcx, pbcy; // pbc -> periodic boundary condition
double dx, dy;
myfile << "d0 is " << box_d0 << endl;
// define the number of atoms as the size of the array of atoms
const int numAtoms = atoms.size();
//pick atoms
for (int i=0; i<numAtoms-1; i++) { // pick one atom -> "Atom a"
Atom &a = atoms[i];
for (int j=i+1; j<numAtoms; j++) { // pick another atom -> "Atom b"
Atom &b = atoms[j];
dx = a.pos.x - b.pos.x;
dy = a.pos.y - b.pos.y;
pbcx = 0.0;
pbcy = 0.0;
// enforce periodic boundary conditions
if(dx > box_LenHalf) pbcx =- box_Len;
if(dx < -box_LenHalf) pbcx =+ box_Len;
if(dy > box_LenHalf) pbcy =- box_Len;
if(dy < -box_LenHalf) pbcy =+ box_Len;
dx += pbcx;
dy += pbcy;
// calculate distance between atoms
dij = sqrt(dx*dx + dy*dy);
// compare dij to the cutoff distance to determine energy
if (dij > box_d0) {
Uij = 0;
} else {
Uij = 1;
}
myfile << "i is " << i << " j is " << j << " dij is " << dij << " d0 is " << box_d0 << " Uij is " << Uij << endl;
UTotal += Uij; // sum the energies
}
}
myfile << "Energy of the system is " << UTotal << endl;
myfile.close();
}
Sorry for the formatting issues - getting the hang of copy/pasting to the forum.