function calculate_tetrahedron(l) {
    var v = [new THREE.Vector3(-l[1]/2,0,0),
             new THREE.Vector3(    0,0,0),
             new THREE.Vector3( l[1]/2,0,0),
             new THREE.Vector3(    0,0,0)];
    v1 = v[1];
    v3 = v[3];
    s0 = l[0]*l[0];
    s1 = l[1]*l[1];
    s2 = l[2]*l[2];
    s3 = l[3]*l[3];
    s4 = l[4]*l[4];
    s5 = l[5]*l[5];
    var x1 = v1.x = (s2-s0)/2/l[1];
    var x3 = v3.x = (s3-s5)/2/l[1];
    var xm = (x1+x3)/2;
    var xyz1s = (s2+s0-s1/2)/2;
    var xyz3s = (s3+s5-s1/2)/2;
    var zs = (xyz1s+xyz3s-s4/2)/2-xm*xm;
    var z = Math.sqrt(zs);
    var z1 = v1.z = ((3*(s2+s0)+(s3+s5)-2*s1-2*s4)/8-x1*xm)/z;
    v3.z = 2*z-z1;
    v3.y = -(v1.y=Math.sqrt(xyz1s-x1*x1-z1*z1));
    return v;
}