Force Field Analysis for Acetylene
The files listed below reproduce the fits given
in Table 1a and Table 1b of L. Hedberg and I. M. Mills, J.
Molec. Spectrosc. 203, 82 (2000). The files are:
- C2H2ff.obs - the observed
quantities for the fit for use with either of the files below.
Points to note:
- Several different (equivalent) ways are used to specify the
- The 13C vibrational frequencies are fitted as
isotope shifts, as these are typically more accurate than
- Different weights (estimated errors ) are used for the
observed values for C2D2, in line with
the original paper.
- C2H2emp.pgo - a fit to
purely empirical symmetry force constants.
- C2H2scaled.pgo - a fit
scaling ab initio force constants to match experiment.
Notes on the data files:
- The force constants are expressed in symmetry co-ordinates,
rather than internal valence coordinates.
- The two degenerate bending modes are repeated with the same
force constant, so F4 and F6 appear twice in the force constant
- The z matrix, shown below, uses Cartesian coordinates
to specify the atomic positions, and has two dummy atoms to
allow the bending modes to be uniquely specified.