BEGIN header

       Real Lattice(A)               Lattice parameters(A)    Cell Angles
   2.4675850   0.0000000   0.0000000     a =    2.467585  alpha =   90.000000
   0.0000000  30.0000000   0.0000000     b =   30.000000  beta  =   90.000000
   0.0000000   0.0000000  30.0000000     c =   30.000000  gamma =   90.000000

 1                            ! nspins
25   300   300                ! fine FFT grid along <a,b,c>
END header: data is "<a b c> pot" in units of Hartrees

 1     1     1            0.042580
 1     1     2            0.049331
 1     1     3            0.038605
 1     1     4            0.049181

int readinputfile() {
   FILE *potential = fopen("orderedfile.txt", "r");
   for (i=0; i<size; i++) {
      fscanf(potential, "%lf %lf %*f  %lf", &x[i], &y[i], &V[i]);
   }
   fclose(potential);
}

int res = fscanf(potential, "%lf %lf %*f  %lf", &x, &y, &v);
if (res == EOF) break;
if (res != 3) {
    fscanf(potential, "%*[^n]");
}

#include <stdio.h>

int main()
{
   /* maybe the buffer must be greater */
   char lineBuffer[256];
   FILE *potential = fopen("orderedfile.txt", "r");

   /* loop through every line */
   while (fgets(lineBuffer, sizeof(lineBuffer), potential) != NULL)
   {
      double a, b, c;
      /* if there are 3 items matched print them */
      if (3 == sscanf(lineBuffer, "%lf %lf %*f %lf", &a, &b, &c))
      {
         printf("%f %f %fn", a, b, c);
      }
   }
   fclose(potential);

   return 0;
}