What You Need to Know to Make a Calculation
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There are two kinds of information you need; 1) the data on the atomic structure itself; and 2) at least preliminary information on interatomic forces.

Regarding data on the atomic structure itself, especially its symmetry, exactly what you need to provide depends on the nature of the structure and the source of information. The main categories, and the dialogs in the Input Menu in which they are entered are

1) Structure axes and lattice translations. For a crystal, the unit-cell axis lengths and angles are required. For a molecule, Cartesian axes are normally used. You can do a calculation for a crystal by using lattice translations in all three axis directions; for a molecule by using no lattice translations; or for a polymer by using translations in one or two directions. (Title/Axes dialog)

2) Symmetry. You need to know the space group for a crystal or polymer or the point group for a molecule. Of course, if you are not interested in symmetry, and you know the location of all atoms, the calculation can be made disregarding symmetry, but this is generally slower and takes more computer memory. (Symmetry dialog)

3) Rotations required to attain standard orientation (see Orientation and Rotations). Some space groups are oriented in a way which is not consistent with the basis functions used for symmetry analysis. A rotation of both the symmetry matrices and the atoms of 30 or 45 degrees about the z axis may be required. However, this is normally done automatically when you specify the space group. (Pre-Calculation Rotations dialog)

4) Coordinates for a symmetry-unique set of atoms, or for all atoms. These are fractional in terms of the unit-cell axes for a crystal, and normally Cartesian for a molecule (Input Atoms dialog). Each atom is given a type, which is typically the atomic number.

5) Types of atoms present. For each atom type used in the preceeding category, it is necessary to specify the mass, and also the charge for calculating infrared intensities. (Atom Types dialog). For natural isotopic mixtures the standard atom list may usually be used, but if particular isotopes are used it may be necessary to use their atomic weights.

If you import data for a crystal from one of the supported data-base formats, all this information, except possibly 3), is likely to be supplied automatically in the file. For a molecule with no symmetry, default values can be used for the symmetry and structure axes. In most cases, the atoms are referred to by their atomic numbers, and the masses and charges are obtained from a standard table. However, in some cases, for example calculating structures with isotope substitutions, it will be necessary to modify the types of atoms.

Forces are entered through the dialogs of the Forces menu, or in the case of manual forces, by clicking with the mouse on the atoms involved. Determining the interatomic forces is the object of the calculation, so exact values are not normally available for input - what you must supply are educated guesses. Atoms supports valence forces of the standard types, namely bonds, three-atom angles, four-atom torsion (tau) or bond-plane (psi) angles, and interactions of bonds and three-atom angles. Apart from cases in which certain vibrations are determined uniquely, which are occur only for the simplest structures, you should look at previous calculations on similar structures to find reasonable starting values. Force constants are usually strongly dependent on bond lengths, and to a lesser extent, angle values.

If you are not very familiar with the structure, it may be very helpful to use the Coordination option in the Input Atoms dialog. This gives a printout of the possible bonds and (3-atom) angles involving each atom, and also a sorted list of bonds and angles.