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 */ }