void Driver::generate()
{
  char str[MAXLINE];
  int leading_dir = 1;
  printf("\n"); for (int i=0; i<70; i++) printf("="); printf("\n");
  while (1){
    printf("Please input the extensions in x, y, and z directions: ");
    if (count_words(fgets(str,MAXLINE,stdin)) < 3) continue;
    nx = atoi(strtok(str,  " \t\n\r\f"));
    ny = atoi(strtok(NULL, " \t\n\r\f"));
    nz = atoi(strtok(NULL, " \t\n\r\f"));
    natom = nx*ny*nz*latt->nucell;
    if (natom > 0) break;
  }