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{ IC } ACCUM [COMPARE] [ADD] {COOR} FIRSTU unit-number [NUNIT integer] {VEL } [BEGIN integer] [STOP integer] [SKIP integer]

The function of the `ACCUM` command is to accumulate
averaging information for the internal coordinates, positions, or
velocities. The `ACCUM` command can add to existing accumulations
or initiate a new set of accumulations. It will also perform the
principal axis transformation on positional or velocity moments
automatically once the accumulation is complete.

The data used by `ACCUM` is produced during the dynamics
calculations. Two sets of data are produced, the coordinates and
velocities of all the atoms at periodic intervals during the runs. One
specifies a set of these files by specifying the first unit number of
the first file and specifying how many unit numbers after that should
also be read. These files should be arranged in time order.

To use the `ACCUM` command, two operands must always be
specified — the first unit from which the coordinate or velocity sets
will be read and which data structures the accumulations should go
into. `FIRSTU` specifies the unit number of the first unit. `IC`
specifies that the internal coordinate averages are to be accumulated;
the files which are read must be dynamical coordinates. `COOR`
specifies that the coordinates averages are to be accumulated; the files
must dynamical coordinates. `VEL` specifies that velocity averages
are to be accumulated; the files must be dynamical velocities. If the
files do not have what they are expected to have, the analysis facility
will stop execution of CONGEN. The files themselves may be read
repeatedly.

The `NUNIT` option specifies how many units are to be read
for the accumulation. If not specified, it defaults to 1.

The `COMPARE` option specifies that the accumulation should
be done using the comparison structure's averages. This allows
comparisons between dynamical calculations. If not specified, the
accumulation is done with respect to the main structure.

The `ADD` option allows one to add to a preexisting
accumulation. If not specified, the accumulation is started from
scratch. When it is specified, the data in the files is added to what is
already present in the accumulation. Some important conditions must be
satisfied for this to be done. First, the sampling interval for the
pre-existing accumulation and the added data must be the same. The
sampling interval is determined by the `SKIP` option (see below).
Next, the range of the dynamics steps covered by the pre-existing
accumulation and the files to be read must be adjacent in time. In other
words, the stopping step of one set plus the sampling interval must
equal the beginning step of the other set. These conditions ensure that
the averages will be meaningful.

The `BEGIN` option specifies the first step in the files to
be accumulated. All records which have a step number greater than or
equal to `BEGIN` will be accumulated. If not specified, it defaults
to 1 so that the range of steps to be accepted is not bounded from
below.

The `STOP` option specifies the last step in the files to be
accumulated. All records which have a step number less than or equal to
`STOP` will be accumulated. If not specified, it defaults to
infinity so that the range of steps to be accepted is not bounded from
above.

The `SKIP` option indirectly specifies the sampling
interval. As the coordinates or velocities are read in, the step number
modulo `SKIP` is computed. If this number is zero, the step is used
in the accumulation. If the step number modulo `SKIP` is not zero,
the step is ignored. If not specified, `SKIP` defaults to 1. One
must careful when using this option. If coordinates were written every 5
steps of dynamics and one specifies `SKIP` to be 7, the sampling
interval will be 35 steps.

Reorienting a coordinate trajectory is possible using the COMPARE command. For details see Reorienting a Coordinate Trajectory.