  We consider mechanically generated molecular braids composed of two molecules
where long range interactions between them can be considered to be very weak.
We describe a model that takes account of the thermal fluctuations of the
braid, steric interactions between the molecules, and external mechanical
forces. In this model, both sets of ends, of the two molecules, are considered
to be separated by a fixed distance apart much larger than the radius of the
braid. One set of ends are rotated to generate a braid of a certain number of
pitches (or turns), while the other set remains fixed. This model may describe
the situation in which the ends of each molecule are attached to a substrate
and a magnetic bead; to the latter a pulling force and rotational torque can be
applied. We discuss various aspects of our model. Most importantly, an
expression for the free energy is given, from which equations, determining the
various geometric parameters of the braid, can be obtained. By numerically
solving these equations, we give predictions from the model for the external
torque needed to produce a braid with a certain number of turns per bending
persistence length, as well as the end to end extension of the two molecules
for a given pulling force. Other geometric parameters, as well as the lateral
force required to keep the ends of the two molecules apart, are also
calculated.
