Seriously desperate as deadline is the end of the week. I am having a complete nightmare with my code to do a 4th order Runge-Kutta to get trajectories for electron paths. I have written some code that at least outputs something, however if I try and vary ANYTHING it cocks up.

Basically I want to change my lower limit to -0.003, my upper limit to 0.007.

Initial conditions:

y(x=-0.003)=a

dy/dx|(x=-0.003)=0

My original equation is

d^2y/dx^2 + ((e[B0exp(-x^2/0.000001)])/(8mV))

where B0 is 1, 5 or 15

Help!!!

Code:

#include <stdio.h>
#include <math.h>
#define N 2 /* number of first order equations */
#define dist 0.1 /* stepsize in t*/
#define MAX 10 /* max for t */
#define charge 1.60217646e-19 /* elementary charge*/
#define mass 9.1093897E–31 /* rest mass of electron */
FILE *output; /* internal filename */
main(){
double t, y[N];
int j;
void runge4(double x, double y[], double step); /* Runge-Kutta function */
double f(double x, double y[], int i); /* function for derivatives */
output=fopen("traject.dat", "w"); /* external filename */
y[0]=0.001; /* initial position */
y[1]=0; /* initial velocity */
fprintf(output, "-3\t%f\n", y[0]);
for (j=1; j*dist<=MAX ;j++) /* time loop */
{
t=(j*dist)-3;
runge4(t, y, dist);
fprintf(output, "%f\t%f\n", t, y[0]);
}
fclose(output);
}
void runge4(double x, double y[], double step){
double h=step/2.0, /* the midpoint */
t1[N], t2[N], t3[N], /* temporary storage arrays */
k1[N], k2[N], k3[N],k4[N]; /* for Runge-Kutta */
int i;
for (i=0;i<N;i++) t1[i]=y[i]+0.5*(k1[i]=step*f(x, y, i));
for (i=0;i<N;i++) t2[i]=y[i]+0.5*(k2[i]=step*f(x+h, t1, i));
for (i=0;i<N;i++) t3[i]=y[i]+ (k3[i]=step*f(x+h, t2, i));
for (i=0;i<N;i++) k4[i]= step*f(x+step, t3, i);
for (i=0;i<N;i++) y[i]+=(k1[i]+2*k2[i]+2*k3[i]+k4[i])/6.0;
}
double f(double x, double y[], int i){
if (i==0) return(y[1]); /* derivative of first equation */
if (i==1) return(-y[1]-y[0]);
//if (i==1) return(-(charge*(exp(pow(-t,2)/pow(-.001,2))/(18*1e5))*y[1]-y[0]);
/* derivative of second equation */
}