# Command syntax

This chapter describes how the commands that are used to define a CAC simulation are formatted in a CAC input script cac.in.

In a CAC simulation, default settings for some commands are first established by defaults.f90, then the entire cac.in is read to override some of the default settings: (i) a blank line or a line with the "#" character in column one (a comment line) is discarded, and (ii) each command should contain no more than 200 characters. Subsequently, input_checker.f90 is run to check whether all commands that do not have default settings are provided in cac.in. In preparing cac.in, it is important to follow the syntax and to distinguish between an interger and a real number, e.g., a real number must be written as 2. or 2.0, instead of 2.

The sequence of the commands in cac.in does not matter, except for the modify, group, fix, and cal commands, in which case extra commands that (i) appear later and (ii) exceed the numbers in modify_number, new_group_number, fix_number, and cal_number, respectively, will be ignored. For example, if cal_number = 2, the last cal command below will be ignored:

cal first_group energy
cal another_group force
cal last_group stress


During the CAC simulation, the user may get a self-explanatory error message, followed by termination of the program by:

call mpi_abort(mpi_comm_world, 1, ierr)


if something is potentially wrong or a warning message.

When boolean_restart = t, the elements/nodes/atoms are read from the cac_in.restart file, in which case all commands in the Simulation Cell category below become irrelevant; otherwise, the simulation cell is built from scratch.

Below is a list of all 34 CAC commands, grouped by category.

• Simulation Cell

• Materials

• Settings

• Actions

• Miscellanies