Relaxed PES scan using Firefly

I'm interested in analysing the barrier to rotation around a particular bond in a molecule. A simple way to look at this is to fix the torsion angle around the bond, and optimise the other coordinates. The end result is a plot of the energy versus the torsion angle.

Here I'll look at the energy profile when rotating around the C-C bond in ethane. There seem to be two approaches to do this using Firefly. The first is described at the Firefly website and involves using IFREEZ in the STATPT group.

However, this approach seems to require users to spell out the internal coordinates themselves. Instead, it's easier to have Firefly work out the coordinates itself. After some hacking based on examples in the docs (see links below), I came up with the following which appears to give the same results but I think is more generally useful:

 $CONTRL RUNTYP=rsurface NZVAR=18 SCFTYP=RHF COORD=CART UNITS=BOHR
         MAXIT=50 $END
 $SYSTEM TIMLIM=500 MEMORY=30000000 $END
 $BASIS  GBASIS=n31 ngauss=6 $END
! To scan along the torsion 6,2,1,4 from 0->120 in steps of 4 
 $zmat dlc=1 auto=1 ifzmat(1)=3,6,1,2,4 scan=.t. DLCTOL=1D-7 ORTTOL=1D-7
       ifdmod=2 $end
 $surf ndisp1=31 disp1=4 orig1=0 reuse=.t. $end
 $DATA
CH3-CH3
C1
 C           6.0     0.0000000000        0.0000000000       -1.4550890105
 C           6.0     0.0000000000        0.0000000000        1.4550890105
 H           1.0     1.9475804164        0.0000000000       -2.1256947270
 H           1.0    -0.9737902082       -1.6866541165       -2.1256947270
 H           1.0    -0.9737902082        1.6866541165       -2.1256947270
 H           1.0    -0.9737902082       -1.6866541165        2.1256947270
 H           1.0     1.9475804164        0.0000000000        2.1256947270
 H           1.0    -0.9737902082        1.6866541165        2.1256947270
 $END

Note: the IFDMOD=2 was added as suggested on the Firefly mailing list, as I originally got the error message "UNABLE TO PROJECT DLC!".

Docs: [Overview] [Full list of options]

Notes to self:
(1) Firefly was run on Windows as follows:

set PATH=C:\Tools\firefly;%PATH%
set INPUT=myinputfile.txt
firefly -f > myoutputfile.txt

(2)For the actual system of interest, I found that the initial conversion of the 'displacement' from internal to cartesian coordinates failed to converge until I set the starting angle for the scan to close to the angle in the input data.
(3) The graph was created using the following Python code: