Collision strengths are calculated using the Breit-Pauli version of the R-matrix method (Scott & Taylor 1982), following a similar procedure to that of Berrington (1988), who studied atomic oxygen.
The target wavefunctions are constructed from 1s, 2s, 2p, 3s, 3p, 3d, 4s,
and 
orbitals;
The 1s through 4s radial orbitals are from Clementi & Roetti
(1974), optimised on the 
ground state.
The 
correlation orbital
is optimised on the 
states using Hibbert's
(1975) program CIV3, in order to account for the different radial
distribution of the d-orbital in the ground and excited states.
This improved the term energy splitting of the states.
Long-range polarization effects are included by introducing the pseudostates
(
, 
,
, 
),
each of which involves a 
pseudo-orbital.
The first three pseudostates are
chosen to represent the dipole polarizability of the 
ground state;
that is, the 
orbital and the pseudostate eigenvectors are
optimised on the polarizability using program CIVPOL (Hibbert et al.
1977). The fourth pseudostate, 
, is
inserted to include as much as possible of the dipole polarizability of the 
metastable state. With these pseudostates,
the calculated dipole polarizability of the 
ground state is 92.58 au,
and of the 
state is 61.42 au.
To estimate the true polarizability, two previous R-matrix
photoionization calculations were used, namely the 
targets of
Sawey & Berrington (1992) and of Bautista (1996).
The static dipole polarizability
(calculated using the R-matrix programs: Berrington et al.
1996) of the 
ground state was found to be 104.36 and
103.83 au respectively which shows excellent agreement between both
calculations. This means that 89% of the ground state polarizability is
accounted for in the Fe I target.
The 
and 
orbitals
generated and used for this calculation are tabulated in Table 1 (click here).
The configurations used in each target state are in Table 2 (click here).
There are of course many more configurations allowable in LS coupling
for each symmetry,
but only those making a significant contribution are retained.
These four pseudostates, together with the three physical states
(
, 
, 
)
give rise to 31 fine-structure levels,
which are explicitly included in the R-matrix calculation (see Table 3 (click here)).
Configuration-interaction wavefunctions are used for the target states;
considerable effort is needed to handle the resulting open d-shell
atomic system in the collision calculation.
In particular, (N+1)-electron configurations are restricted to
certain d-shell couplings, such as 
.
The collision strengths were calculated using the FARM packaged developed by Burke & Noble (1995).
Table 1: 
and 
radial orbitals
used in the Fe R-matrix calculation
(the remaining target orbitals are from Clementi & Roetti 1975).
Each orbital is in the form
Table 2: Configurations used in the Fe I target term expansions.
(All 
combinations are assumed unless otherwise stated)
Table 3: Fine-structure energy levels of the
states of Fe I included in the
R-matrix calculation, in Rydberg units.
Observed energies are from Reader & Sugar (1975)