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Three-dimensional Vectors

  The 3d-vector construct allows one to specify a three-dimensional real vector. The construct is also used to specify a 33 matrix by entering one row at a time (see Section 2.4).
( <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.

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