proc rotate_axis {vec deg {molid top}} {
    # get the current matrix
    lassign [molinfo $molid get rotate_matrix] curr
    # the transformation matrix
    set r [trans axis $vec $deg]
    # get the new matrix
    set m [transmult $r $curr]
    # and apply it to the molecule
    molinfo $molid set rotate_matrix "{ $m }"
}