### 9.2 Image Operation

The routines in IMAGES.FLX can be classified into three sections. These
categories are:

- Set up images —
`IMIGIO`

, `IMREAD`

, `IMWRIT`

,
`REIMAG`

, `INIMAG`

.
- Set up energy terms —
`IMHBON`

, `NEWHBL`

, `IMHBFX`

,
`NBONDM`

.
- Compute energy —
`EIMAGE`

, `TRANSO`

, `TRANSI`

,
`SFLSET`

.

The first category involves reading the image file (`IMREAD`

) and
setting up the data structure (`REIMAG`

, `INIMAG`

) see the
CONGEN source file `IMAG.FCM`.

The second category in addition to finding the energy terms,
also selects which image residues are to be kept. This selection process
is repeated each time the nonbonded list is updated. Since the list of
atoms can change its not possible to maintain a fixed list of hydrogen
bonds explicitly for image interactions. It is also the case that the
image Hbond list must be recomputed each time a new nonbond list is
generated. This is done automatically, but it is highly recommended to
keep INBFRQ and IBHFRQ (see General Energy Operands) the same to
avoid confusion. The ST2 interactions are also computed when the
nonbonded list is updated (`NBONDM`

).

The third category is concerned with the computation of energy
terms. For the actual computation of energy, standard routines are used
(`ENBOND`

, `EHBOND`

, `ENST2`

) with a modified calling
sequence. The procedure used is:

- Compute coordinates for all image atoms.
- Set up arrays for self energy terms (atom with its own image).
- Compute self terms, divide energy by 2, zero out image forces.
- Compute remaining terms including forces on image atoms.
- Transform forces on image atoms back into the primary space.

Using a procedure where the forces on image atoms is kept, allows for a
substantial reduction in the number of necessary image atoms. This
results in the necessity that all transformations have an inverse. This
procedure has the drawback that the self energy terms must be treated
specially and that all hydrogen bonds between image and primary atoms
must be computed and then trimmed of any repeats.
Since there is no treatment of the second derivative of the
energy for image atoms, the procedures involving Newton-Raphson
minimizations (see Minimization) and vibrational analysis
(see Vibrational Analysis) will not function properly.