veccross implementation in tclforces

From: Daniel Strahs (
Date: Wed Sep 11 2019 - 16:55:53 CDT

Hello all:

I'm implementing a tclforces script to rotate part of a protein around
a calculated axis of rotation.

The script, based on the rot-a.tcl script from the forces tutorial,
calculates the axis of rotation (R) at each step, and then solves for
the vector (A) from the atom to the rotation axis that is perpendicular
to the rotation axis. The direction of rotation is then the cross
product of R x A.

In implementing this script, I used veccross to determine the RxA
vector product. Unfortunately, the simulation fails immediately with
the error FATAL ERROR: wrong # args while executing "veccross
$comvector $atom2rotvec". In this example, comvector is R and
atom2rotvec is A. Both comvector and atom2rotvec are vectors.

I edited the script slightly to test it with VMD (getting coords and
masses using atomselect), and the script works fine. It computed a
rotational force for each atom that produced the correct rotation, and
veccross has no problems with it's input.

Is veccross implemented in tclforces? I could always compute the cross
product with brute force explicit sums, but I did like the elegance of

Thanks for any pointers!

Daniel Strahs
Biology Dept
Pace University

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