Three-dimensional Vectors

**3d-vector:==**-
**( real real real )**- specifies the vector explicitly by using the x,y,z components.
**(****[TAIL=selection] [HEAD=selection] )**defines the tail or the head of the vector as the center of mass of the specified atom selections using the mass atom property (Section 2.16) and the main coordinate set x,y,z (Section 6.1). If HEAD is not specified, it defaults to (0,0,0); similarly for TAIL.