Contents: Table of Contents   Runtime Limits  Chapter 1  Limitations

Overview of input

The data input to CADPAC is organised in terms of directives which take three forms: those specifying wavefunction type (e.g. RHF, MP2), those which modify default settings such as convergence and those which specify the desired calculation type( e.g. ENERGY, or OPTIMIZE). The directives associated with setting parameters can be of single-line form or multi-line form terminated by the keyword END.

The structure of a typical dataset would be of the form

RUN PARAMETERS	(Optional) e.g. MEMORY, TIME
TITLE	One-line description of your choosing.
GEOMETRY PARAMETERS	Relevant parameters related to the
	geometry such as the directives
	ANGSTROM, SYMMETRY,
	CONSTANTS, VARIABLES,
	BASIS, DIHEDRAL .
GEOMETRY SPECIFICATION
WAVEFUNCTION SPECIFICATION
ALL OTHER DIRECTIVES
START	indicates that the calculation is to start
FINISH	Signals the end of the dataset.

Examples of various datasets appear throughout this chapter. However, to provide a concrete example now, here is a simple dataset,
 

TITLE
NH2 631G* uhf
SYMMETRY
CNV 2
END
BASIS 631G*
VARIABLES
  NH   1.01 A
  ZNH  50.0 D
END
ATOMS
Nitrogen  7  0.0   0.0   0.0
Hydrogen  1   POL   NH  ZNH
END
UHF
MULTIPLICITY 2
ENERGY
START
FINISH

This contains various directives explained later, but the structure corresponds to the general form given above - there are parameters specifying the geometry, and the basis set. The geometry in this example is in polar coordinates which is only useful for small molecules, and a more general internal coordinate input is described later. It is also possible to use simple cartesian geometries. The dataset also contains the type of wavefunction (UHF) and the type of calculation (ENERGY). The calculation commences when the START directive is encountered, and finishes when FINISH is found. If you insert further commands between the START and FINISH then further calculations continuing from the end of the energy calculation will occur. Note the use of END to terminate multiline directives. Two further points about datasets in general - all input is in free format and all commands must be in upper case. We shall now work through the possible commands in roughly the order they occur in the example dataset.