float getchivolp(point a1,point a2,point a3,point a4)

//  see Crippen & Havel book, p. 27.
{
	return ((a1.z*a2.y*a3.x
		-a1.y*a2.z*a3.x
		-a1.z*a2.x*a3.y
		+a1.x*a2.z*a3.y
		+a1.y*a2.x*a3.z
		-a1.x*a2.y*a3.z
		-a1.z*a2.y*a4.x
		+a1.y*a2.z*a4.x
		+a1.z*a3.y*a4.x
		-a2.z*a3.y*a4.x
		-a1.y*a3.z*a4.x
		+a2.y*a3.z*a4.x
		+a1.z*a2.x*a4.y
		-a1.x*a2.z*a4.y
		-a1.z*a3.x*a4.y
		+a2.z*a3.x*a4.y
		+a1.x*a3.z*a4.y
		-a2.x*a3.z*a4.y
		-a1.y*a2.x*a4.z
		+a1.x*a2.y*a4.z
		+a1.y*a3.x*a4.z
		-a2.y*a3.x*a4.z
		-a1.x*a3.y*a4.z
		+a2.x*a3.y*a4.z)/6.);
};

float getchivol(molecule m, string aex1, string aex2, string aex3, string aex4)
{
	point	p1, p2, p3, p4;

	setxyz_from_mol( m, aex1, p1 ); 
	setxyz_from_mol( m, aex2, p2 ); 
	setxyz_from_mol( m, aex3, p3 ); 
	setxyz_from_mol( m, aex4, p4 ); 
	return( getchivolp( p1, p2, p3, p4 ));
};