Building and Optimizing BF3

Start Moilin

Click to go into build mode and click the screen.

On the Build Options dialog set the atom type to B (case does not matter) and select the geometry shown. It is best when dealing with an unknown structure to start from the lowest possible symmetry. Click OK and BH3 appears

To convert the structure to BF3 click one of the H atoms

and on the Build Options dialog set the Atom Type to F and the geometry to monovalent. Repeat this for the other H atoms.

Right click (click the right mouse button) to return to display mode and click to open the Mopac menu.
Click NewInput and

click Run Mopac on the Mopac options dialog. 
This will optimize the structure.

You will see Mopac finished structure updated on the status bar.
Right click to return to display mode.

Rotate the structure by clicking and dragging the mouse. You should be convinced that it is planar.