#include	<stdio.h>
#include	<math.h>
#define	NM	10			/* Maximum number of atoms */
#define	PM	((NM * NM) / 2)		/* Maximum number of pairs */
#define	KB	1.38e-23		/* Boltzmann Constant */
#define	EPS	(100.0 * KB)		/* energy unit */
#define	SIGMA	2.0e-10			/* length unit*/
#define	TDELTA	1.0e-15			/* increment of time step */
#define	WM	6.634e-26		/* mass of atom */
main()
{
	int	i, j, natom, npair, ip[PM], jp[PM];
	double	x[NM], y[NM], vx[NM], vy[NM];
	double	rx[PM], ry[PM], r[PM], rs;
	double	phi[PM], dphi[PM], dphix, dphiy;
	double	fx[NM], fy[NM];
	double	x1[NM], y1[NM], vx1[NM], vy1[NM];

	natom = 10;
	for (i = 0; i < natom; i++) {
		/* Data input */
		scanf("%lf%lf%lf%lf", &x[i], &y[i], &vx[i], &vy[i]);
		/* Initial Data output */
		printf("%2d %15.8e %15.8e %15.8e %15.8e\n", 
			i, x[i], y[i], vx[i], vy[i]);
	}

	/* Finding pairs between atoms */
	npair = 0;
	for (i = 0; i < natom - 1; i++) {
		for (j = i + 1; j < natom; j++) {
			rx[npair] = x[i] - x[j];
			ry[npair] = y[i] - y[j];
			r[npair] = sqrt(rx[npair] * rx[npair] 
					+ ry[npair] * ry[npair]);
			ip[npair] = i;
			jp[npair] = j;
			npair = npair + 1;
		}
	}
	fprintf(stderr, "(pair)%5d\n", npair);

	for (i = 0; i < natom; i++) {	/* initialization of force vectors */
		fx[i] = 0.0; fy[i] = 0.0;
	}
	for (i = 0; i < npair; i++) {
		rs = SIGMA / r[i];
		phi[i] = 4.0 * EPS * (pow(rs, 12.0) - pow(rs, 6.0));
		dphi[i] = -24.0 * EPS / SIGMA * (2.0 * pow(rs, 13.0) 
							- pow(rs, 7.0));
		dphix = -dphi[i] * rx[i] / r[i];
		dphiy = -dphi[i] * ry[i] / r[i];
		fx[ip[i]] += dphix;
		fx[jp[i]] -= dphix;
		fy[ip[i]] += dphiy;
		fy[jp[i]] -= dphiy;
	}
	/* New velocities, New coordinates x1, y1, vx1, vy1*/
	for (i = 0; i < natom; i++) {
		vx1[i] = vx[i] + (fx[i] / WM) * TDELTA;
		vy1[i] = vy[i] + (fy[i] / WM) * TDELTA;
		x1[i] = x[i] + 0.5 * vx1[i]* TDELTA;
		y1[i] = y[i] + 0.5 * vy1[i]* TDELTA;
	}
	/* Output */
	for (i = 0; i < natom; i++) {
		printf("%2d %15.8e %15.8e %15.8e %15.8e\n", 
			i, x1[i], y1[i], vx1[i], vy1[i]);
	}
}